air compressor safety valve leaking made in china

If leaking seals and service bills are distressing your compressor, it"s time to face the fix. While air compressors can be as diverse as the individuals that use them, most models share some general characteristics and components. Whether you are inflating your tires or creating empires, eReplacementParts.com provides the parts, procedures and facts you need to fearlessly fix what fails you.

The safety valve is designed to keep the air compressor pump from over-pressurizing the tank. Over time, the spring inside this valve can deteriorate, allowing air to escape at too low of a pressure. Safety valves are preset to very specific pressure tolerances, so it is important to match the specifications of your specific compressor model when replacing the safety valve.

Less than impressed with your compressor? Replacing the safety valve may be the solution. This article will show you how to complete the repair like an expert technician.

Always depressurize the tank before servicing an air compressor. To do this, open the drain valve and wait until all of the air (and pressure) has escaped.

A well-maintained air compressor can mean the difference between performance under pressure, and under-pressured performance. But you don"t have to empty your pockets to keep your air tank full. As you just learned, repairing your air compressor is simpler than you think, especially when you follow our step-by-step guides. Not only did you refresh compression at a fraction of the cost of replacement; you have inflated your ability to fearlessly face the next fix, regardless of the pressure involved.

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When the system is running, the motor runs the compressor, and the compressor sucks in filtered air, compresses it, sends it to the refrigerator, and finally to the air receiver tank. At this stage, the minimum pressure valve is energized (power on), the circuit is closed, and the pressure is maintained. When the fuel tank reaches the required pressure, the pressure switch stops the motor, and the non-return valve (Check valve) keeps the air in the fuel tank. It prevents the compressor from being damaged by keeping it under pressure for too long. Then the solenoid valve is de-energized (power off) and the air in the air compressor is to be discharged. Learn more about all types of valves in air compressors.

After the compressed air is discharged from the compressor, it needs to be transported to the place of use through pipelines. If there is an air leakage problem in the pipeline at the use site, it will increase the energy consumption of the air compressor and cause unnecessary waste. Therefore, controlling leakage is an important way to save energy for screw air compressors.

Reason: The supporting surface of the safety valve spring is not perpendicular to the spring centerline. When the centerline of the valve spring is not perpendicular to the supporting surface, and the valve spring is compressed, it will deflect, causing uneven force on the disc of the safety valve, warping, causing air leakage, oscillation, and even failure of the safety valve.

Solution: Adjust the verticality of the spring support surface of the safety valve to the centerline of the spring, keep them perpendicular to each other, clean up the impurities and dirt between the safety valve and the valve seat, and re-grind if necessary to ensure a tight contact surface. The valve spring of the safety valve should be pressed tightly, the thread and the sealing surface should be protected, and the damaged part should be repaired and ground.

Reason: The cause of air leakage is often due to poor quality of selected materials and accessories or installation quality. The sinking of the bracket in the air compressor pipeline causes severe deformation and cracking of the pipeline, and the accumulation of water in the pipeline will cause the pipe or fittings to burst and crack.

Solution: Repair or replace damaged pipe sections and fittings, regularly remove the water in the pipeline to prevent freezing and cracking; when the pipeline support sinks, the support should be repaired, and the slope of the pipeline should be adjusted to facilitate drainage.If necessary replace the pipeline with other better materials like aluminum alloy or stainless steel.

In addition, due to too many elbows in the compression system, the airflow resistance increases, and additional work points are formed. At the same time, gas shocks will be formed at the elbows, local pressure will increase, and it will continue to operate at high air pressure. Easy to uninstall. Optimize the system piping, try to avoid using elbows, and reduce energy consumption.

Many users have invested a lot of energy and money in the selection, operation, and maintenance of compressor equipment, but due to negligence on the compressed air pipeline, a lot of waste has been caused. What I recommend here is an economical and simple maintenance method.

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The main parameter of the air compressor safety valve is the displacement, which is determined by the diameter of the valve seat and the opening height of the valve disc.

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Conventionally when we talk about oil lubricated screw air compressor maintenance, it is mostly about replacing consumables such as filters and lubricant on time. While these consumables have a defined usable life and have a direct effect on the efficiency and the life of the air compressor itself when not replaced on time, there are a few critical valves in the air compressor that require maintenance as well. Compressor valves directly affect the efficiency, safety, and the functionality of the screw air compressor. Let us understand some of the commonly available valves in a screw air compressor, why they need maintenance, and discuss some of the frequently asked questions about screw air compressor valves.

A screw air compressor is very similar to a human heart. While a human heart has tricuspid, pulmonary, mitral, and aortic valves, a screw air compressor has four critical valves namely air inlet, minimum pressure, blow down, and safety valves.

Air inlet valve is also commonly known as the ‘Intake valve’ which is typically assembled on the airend’s intake. The air inlet valve of a conventional fixed speed screw air compressor controls the air intake into the compressor. It remains closed when the compressor starts to lower the starting load on the main motor and when the desired working pressure is attained in the compressed air circuit and thus enabling the compressor’s motor to run without any load. In some compressors that are capable of providing a variable output by modulating the amount of air it sucks in, the inlet valve holds various opening positions to regulate the volume of air entering the compressor. The effective performance of the inlet valve directly affects the compressor’s capacity and its power consumption during load and no-load conditions.

The minimum pressure valve is typically assembled on the exit of the air-oil separation tank of a compressor. The minimum pressure valve acts as a check valve preventing back flow of compressed air into the airend, retains a minimum pressure in the compressor system for lubrication, offers a restriction to avoid a collapse of the air-oil separation filter, and ensures a suitable velocity of flow across the air-oil separator that ensures efficient air-oil separation. The effective performance of the minimum pressure valve directly affects the compressor’s lubrication, air-oil separation efficiency, and power consumption during load and no-load conditions.

The blow down valve is typically found on a dedicated exhaust line from the air-oil separation tank. The blow down valve evacuates the compressed air in the air-oil separation tank each time the compressor runs on a no-load and when the compressor shuts down to ensure there is no back pressure when the compressor starts to load next time. The blow down valve of a conventional screw compressor is typically actuated by a solenoid valve. The effective performance of the blow down valve affects the compressor’s power consumption during un-load, capacity of the compressor when running on load, and the life of the motor.

The safety valve is typically mounted directly on the air-oil separator tank. The only function of the safety valve is to blow off the compressed air in the air-oil separation tank when the pressure in the air-oil separation tank exceeds the set pressure of the safety valve and there by prevents the tank from cracking under high pressure. A malfunctioning safety valve affects the safe operation of the air compressor or results in leakage of compressed air continuously.

Though each compressor manufacturer has their own unique valve design, compressor valves in general contain moving parts such as springs, valve plates, and plungers that affect the opening and closing of the valves and rubber seals / seats that offer perfect sealing when the valves remain closed. These moving parts wear or lose their mechanical properties over a period of time and the sealing components typically ‘age’ over time and lose their effectiveness and will need to be replaced.

Compressor manufacturers typically design these components to operate efficiently for several thousand or millions of operation cycles. However, several factors such as variability in the demand pattern, sizing of the air compressor against a certain air demand, the environment in which the air compressor operates, promptness of preventive maintenance, etc. determine how long these valves efficiently operate.

Many times, it is difficult to identify a malfunctioning valve or a valve operating with worn-out parts as the compressor continues to generate air. The typical symptoms of a malfunctioning valve are loss in compressor"s capacity, increase in power consumption during load or/and unload, drop in discharge pressure, increase in oil carry-over and more load on motor. These symptoms are either difficult to notice or have other frequently common assignable causes such as air leak before suspecting the compressor valves.

Case studies show that operating a screw air compressor with a worn-out / malfunctioning valve could increase its overall power consumption by 10 - 15%. Power cost contributes to more than 75% of the compressor’s total life cycle cost over ten years and hence this is a significant impact. Unserviced valves also lower the life span of downstream accessories by half. In some cases, a malfunctioning safety valve may result in a catastrophe.

Air compressor manufacturers typically offer convenient valve maintenance kits for customers that contain the internal parts of the valve that wear or age out. Changing the valve kits is a much more sensible and economical option than changing the complete valve.

It is difficult or almost impossible to identify a malfunctioning valve unless it is opened for inspection. Hence it is absolutely mandatory that these valves are inspected for effectiveness every year and the internal moving parts replaced as a part of preventive maintenance once every year or two depending on the operating conditions of the air compressor. It is typical for compressor manufacturers to mandate a valve kit replacement once every two years as a proactive measure.

In particular, the safety valve must be inspected and certified every year per the local safety laws to ensure they are functional and efficient. Sometimes, replacing the safety valve entirely with a valid certificate for one year is more economical as the certification procedures could be equally expensive on an existing valve.

As stated before, it is challenging to identify a valve that is worn out unless it is opened and inspected, but there are a few indicators that a qualified compressor technician can use to deduct a malfunctioning valve.

Low duty cycle operation: A sophisticated screw air compressor in today’s day and age carries a convenient microprocessor-based human-machine interface that keeps track of operating hours of the compressor under load and un-load conditions and the number of load/unload counts the compressor is subjected to over a period of time. A higher un-load hours and load/unload count indicates that the air compressor is oversized against the actual air demand. This in turn indicates the air compressor ‘cycles’ frequently between load and un-load mode as opposed to running continuously on load. Every time a compressor ‘cycles’, the inlet valve, blow down valve, and minimum pressure valve is brought into play where their internals ‘actuate’. Frequent actuation of these valves results in a faster wear of the internals and hence results in shorter life.

High operating temperature: A compressor that runs on a high operating temperature affects the life of the valve’s sealing components, which causes them to ‘age’ fast.

Compressor not building pressure: If the air demand has not changed over time and the facility is relatively free of any air leakage, the air compressor is probably not delivering the rated output. There is a high probability that there is a malfunctioning valve.

Increase in compressor’s power consumption: An increase in the air compressor’s power consumption profile over a period of time where there has been no abnormal change in the air demand and usage pattern indicates an increase in either the load or un-load power. There is a high probability that this is because of a malfunctioning valve.

Based on the design philosophy adopted by the air compressor manufacturer, the oil lubricated screw air compressors could have a few more valves that are critical to functional performance that must be maintained as well. Some of the other valves frequently used in an air compressor are as follows:

Temperature control valve (also known as thermal valve) is used to regulate the flow of oil through the oil cooler based on the operating temperature.

Drain valves are used to drain lubricant at the time of lubricant change over or cleaning. Air compressors equipped with a moisture trap at the outlet of the after cooler also has a drain valve (automatic or timer based) to discharge water collected

The presence or absence of one of these valves and the type of actuation of these valves (electronic / mechanical) depends on air compressor’s design architecture. The Operation and Maintenance Manual (OMM) and the Piping and Instrumentation Diagram (P&ID) supplied by the air compressor manufacturer are excellent resources that explain the purpose, functioning, and maintenance requirements of these valves.

Many of the air compressor valves are highly specialized and exclusive. Their designs are usually complex and some even need special tools to service them. The internal components" build quality and material selection are extremely important and proprietary. Hence it is highly critical that only genuine valve kits issued by the air compressor manufacturer are used to maintain the valves. An inferior after-market replacement will most certainly compromise the performance of the entire compressor, void the original manufacturer"s warranty of the compressor, cause consequential damage to other parts of the compressor, and above all, be a safety hazard.

In conclusion, while it is important to change the screw air compressor"s filters and lubricants on time, it is equally important to perform preventive maintenance on these critical valves in a screw air compressor as recommended by the air compressor manufacturer. While the intake valve, minimum pressure valve, safety valve, and blowdown valve are critical to the performance and safety of the compressor, there could be other valves in the compressor that are critical and need maintenance. The air compressors sizing and the environment in which it operates are crucial factors that affect the life of the air compressor. Finally, it is critical to proactively service these valves using genuine kits issued by the compressor manufacturer to enable the air compressor performs efficiently and safely.

When it comes to an air compressor, maintenance in the standard form is never enough. To ensure the smooth running of operations and avoid interruptions and unexpected downtime, it is crucial to have a compressor maintenance program in place that prevents — rather than reacts to — problems with the air compressor and all attached tools and machinery.

At facilities that employ high-tech machinery, you must perform more than just standard maintenance to ensure the operability and longevity of all the expensive equipment on hand. With air compressors, for example, you need to implement a preventive maintenance program where you get the compressor and its system peripherals inspected routinely to ensure everything is in optimal running condition.

The purpose of preventive maintenance is to catch mechanical problems before they spread and necessitate costly repairs and system downtime. As such, preventive maintenance consists of inspections of all consequential system components — some according to a daily schedule, others on a weekly, monthly, quarterly or annual basis — to ensure everything works as it should. If you detect problems early, you can take steps to rectify matters.

In some cases, preventive maintenance involves low-cost measures that help avoid costlier situations down the line. For example, when a maintenance staffer notices a compressor belt has a minor crack, replacing the belt then and there helps your company prevent a costlier scenario in which the belt snaps, the compressor stops working and productions grind to a halt as staff work to identify the problem and take the necessary steps to get the system back up and running.

Regardless of the size or scope of your compressor operation, it is vital to have someone on staff, or work with a qualified service provider, to oversee preventive maintenance work on a timely and consistent schedule. Only with preventive maintenance can you ensure the machines will work hour by hour, day after day, and continue to operate to their full life expectancy.

One of the foremost benefits of air compressor maintenance is that it makes the equipment run smoothly and more efficiently and decreases occurrences of system downtime. As anyone who runs an air compressor facility knows, downtime can be a very costly problem because when the machines fail to function, it halts production.

With reduced occurrences of downtime and parts failure in your air compressor system, you save money. As such, you profit both ways — through increased productivity and reduced overhead. The money you save through timely compressor maintenance allows you to invest more in better equipment as innovations reach the market.

Without maintenance, the costs involved in the repair of an air compressor and its peripheral parts can take a huge chunk from a company’s annual earnings. When you conduct timely maintenance, you’ll reap savings you can pass on to your company through higher wages and less expensive products.

When you perform air compressor maintenance on a regular schedule, it allows you to catch instances where a function within the system is over-exerting or struggling to maintain an expected rate of production. When such issues do arise, it is often down to a part that needs cleaning, replacement or lubrication. By spotting these problems before they grow out of hand, the machine runs smoother and more efficiently, which translates to energy savings.

Of course, the greatest benefit of air compressor maintenance is that it increases the life and efficiency of the machine itself and the system as a whole. When you add up the initial cost of investment in an air compressor and all the attached pneumatic tools, you want to ensure a return on that investment through years of optimal performance. Ideally, the money you spend on your compressor system should reward you thousands and thousands of times over through productivity.

Without maintenance, an air compressor and its peripheral components will not last as long as it otherwise could with regular checkups, tune-ups and cleanups. When you compare the profits of companies that implement responsible system maintenance with those that do not, you are liable to see major differences in productivity.

On an industrial air compressor, preventive maintenance is crucial to ensure the functionality of the system and its various attachments. The key parts to check include the filters, vents, belts and bearings, all of which could become troublesome to the system if dirt and grime build up. Moreover, you must apply and reapply lubricant at timely intervals on all applicable parts of an air compressor.

The purpose of an air compressor is to produce clean, pure, compressed air that will ultimately power numerous functions. To ensure the quality of air that comes out at the end, the ambient air that goes into the compressor must be filtered of impurities before it leaves the machines. None of that could be possible without a clean air filter.

If the air filter is dirty, impurities and particulates could corrupt the compressed air and degrade the quality of end-point applications. Therefore, clean the air filter regularly. Change it out at regular intervals, which vary based on the environment.

Oil can degrade the quality of compressed air if it passes through the system and gets carried to the end of an application. Some of the worst-affected processes would include pneumatic spray painters, air cleaners and anything else where oil could corrupt the surface in question. Therefore, it is crucial to ensure oil, when present in the system, is removed from the compressed air before the air leaves the machine.

Check oil filters weekly, regardless of whether the compressor is lubricated or non-lubricated. Moreover, replace the oil filter entirely at recommended intervals, which can range from 4,000 to 8,000 hours of use depending on your unit. If the oil filter gets heavily covered in oily residue before that time, replace it sooner.

Lubricant is one of the most vital elements in the function of an air compressor. On all the internal metal parts and joints, lubricant allows for smooth, non-corrosive movement. Without lubrication, tension occurs between the touching metal surfaces, which leads to the corrosion of parts and joints. Once corrosion takes hold, rust is liable to spread and eat through certain mechanical parts.

However, even when lubricant is present, it can lose its viscosity and become corrosive if it gets too old. Check the lubricant level daily to ensure the health of your air compressor. Every three to six months, wipe off old lubricant and reapply a fresh coat. Each time you replace the lubricant, be sure you also change out the separator element.

To protect the health and performance of the air compressor motor, grease the bearings every 4,000 hours. Be sure to inspect the bearings at quarterly intervals between each greasing to ensure they remain sufficiently lubricated.

For an air compressor to go about its internal motions, it is crucial for the belts to have proper tension. The rubber of each belt must also remain firm, yet flexible, to ensure balanced movement between the pulleys of connected parts. Over time, however, the rubber on a belt will inevitably wear down and crack in certain places. Therefore, it is crucial to replace the belts before they lose their tension or, even worse, snap in the middle of an operation.

An air compressor performs the magic feat of transforming ambient air into something that can power heavy-duty machinery and effectively serve as a replacement for electrical power. That said, the compressor itself can only do so much to turn mundane air into something powerful. While internal components do their job to purify the air for end-point use, that job is harder for the machine to perform if the intake vents become lined with dirt and grime.

To ensure the incoming air remains as clean as possible and to prevent dirt from getting sucked into the system, inspect the intake vents weekly and clean them when necessary.

In addition to the periodic cleaning, lubrication and replacement of parts, check various points along the air compressor and its attachments at regular intervals. Inspect the following on a weekly basis:

Inspect the air compressor for signs of oil or air leaks. Also check the pneumatic hoses for air leaks, as leakage severely reduces the efficiency of an air compressor. Furthermore, make sure the coolers are free of dirt.

When you make an air compressor preventive maintenance checklist, you need to first take into account the type of compressor in question. Most compressors need preventive maintenance on various system parts at

Company staff at a given facility or plant can generally perform air compressor preventive maintenance in-house. However, it’s better to let professional compressor service people perform some maintenance tasks — even more so if the unit is large or complicated. Unless your company is staffed with highly skilled personnel to handle maintenance tasks with air compressors, it is best to contact a professional for the following:

When you hire a professional for these and other time-consuming and possibly dangerous tasks, it can help you save time and money and also ensure that the job is done properly. Moreover, professional maintenance ensures utmost safety for the more difficult aspects of the job.

To ensure maximum efficiency and an absolute minimum of downtime and repair costs with your air compressor, complete preventive maintenance checklist tasks according to a set schedule. Depending on the needs of a given component, perform maintenance daily, weekly, monthly, quarterly or annually.

When you perform preventive maintenance according to a checklist, your air compressor and its attachments will last longer and perform with greater efficiency. Preventive maintenance makes it possible to detect problems at an early stage before they do serious system damage and lead to costly repairs and downtime. You can invest the money you save with preventive maintenance back into your company’s infrastructure and staff.

For nearly a century, Quincy Compressor has been America’s leading supplier of air compressors and pneumatic tools. Since the 1920s, we have been at the forefront of innovation in air compressor technology. Today, our machines are in office buildings, repair shops, assembly plants and factories throughout the U.S. and abroad.

My Porter Cable Pancake Air Compressor has been a workhorse, flawlessly performing in cabinet trim work and even helping me build a deck frame. Sadly, it started to leak air but rather than visiting a service center/shop ($100 min charge), I decided to see if I can repair it myself.

If you arrived at this page via a search engine, This is a 2 part article. Part 1 shows you how to identify the replacement part. Part 2 shows you step-by-step instruction to replace the faulty compressor drain valve.

I purchased my Porter Cable Pancake Air Compressor as a bundle set for $249. The set included 3 different nailing guns and I have been very happy with its performance.

Although my pancake air compressor is made by Porter Cable (model c2002, type 3*), the information presented here will most likely apply to other brands as well since most of compressors are made by a handful of manufacturers located in Mexico and China.

The compressor’s inability to hold the pressure (PSI) started after I used it heavily to build my deck (read here). Because of tight spaces, I used a compressor-driven palm nailer (worked great) for a period of 2 days out in the sun and the compressor worked really hard.

Faint hissing sounds started soon after with air slowly leaking out. Initially, I thought maybe the drain valve was not closed completely, but within weeks, this hissing sound got louder and louder and my motor was having trouble keeping up with the air leak.

Although there are multiple possible failure points, I decided to check was the drain valve first because this drain valve uses a single compression O-ring to seal the air in. Based on my experience with various plumbing parts, I guesses that this O-ring failed due to heat when I built my deck. Besides, the drain valve is relatively easy and inexpensive to fix!

Option 1 – replace the O-ring (UPDATE: One reader suggested “…a packet of two O-rings…in Canada…O-rings they sell are 1/4 ID, 3/8 OD…They are known as o10 in the O ring industry specifications…; I wasn’t able to find the equivalent one in US, but found this – O-Ring Assortment Kit for Pneumatic Air Rubber Hydraulic Tools)

In the end, I decided to go with a ball valve type drain valve from Bostitch for reliability because I know ball valves are very durable and are not susceptible to heat damage like O-rings.

It also allows me to quickly dump the air out (I personally chose the Bostitch unit over DeWalt/Porter Cable unit because it seems to be made of higher quality material).

If you have attempted to install a new valve but had to remove it several times, the built-in tape is probably no good so it would be best to replace add some new sealant tape

This section tells you about air brakes. If you want to drive a truck, bus, or pull a trailer with air brakes, you need to read this section. If you want to pull a trailer with air brakes, you also need to read Section 6: Combination Vehicles in this handbook.

Air brakes use compressed air to make the brakes work. Air brakes are a good and safe way of stopping large and heavy vehicles, but the brakes must be well maintained and used properly.

CDL Air Brake Requirements. For CDL purposes, a vehicle’s air brake system must meet the above definition and contain the following, which will be checked during the vehicle inspection test:

If the vehicle you use for your road test does not have these components, your vehicle will not be considered as having an air brake system and you will have a “No Air Brakes” (“L”) restriction on your CDL.

A full service brake application must deliver to all brake chambers not less than 90 percent of the air reservoir pressure remaining with the brakes applied (CVC §26502).

The air compressor pumps air into the air storage tanks (reservoirs). The air compressor is connected to the engine through gears or a v-belt. The compressor may be air cooled or cooled by the engine cooling system. It may have its own oil supply or be lubricated by engine oil. If the compressor has its own oil supply, check the oil level before driving.

The governor controls when the air compressor will pump air into the air storage tanks. When air tank pressure rises to the “cut-out” level (around 125 pounds per-square-inch or “psi”), the governor stops the compressor from pumping air. When the tank pressure falls to the “cut-in” pressure (around 100 psi), the governor allows the compressor to start pumping again.

Air storage tanks are used to hold compressed air. The number and size of air tanks varies among vehicles. The tanks will hold enough air to allow the brakes to be used several times, even if the compressor stops working.

Compressed air usually has some water and some compressor oil in it, which is bad for the air brake system. The water can freeze in cold weather and cause brake failure. The water and oil tend to collect in the bottom of the air tank. Be sure that you drain the air tanks completely. Each air tank is equipped with a drain valve in the bottom. There are 2 types:

Some air brake systems have an alcohol evaporator to put alcohol into the air system. This helps to reduce the risk of ice in air brake valves and other parts during cold weather. Ice inside the system can make the brakes stop working.

Check the alcohol container and fill up as necessary. (every day during cold weather). Daily air tank drainage is still needed to get rid of water and oil (unless the system has automatic drain valves).

A safety relief valve is installed in the first tank the air compressor pumps air to. The safety valve protects the tank and the rest of the system from too much pressure. The valve is usually set to open at 150 psi. If the safety valve releases air, something is wrong. Have the fault fixed by a mechanic.

You engage the brakes by pushing down the brake pedal (It is also called a foot valve or treadle valve). Pushing the pedal down harder applies more air pressure. Letting up on the brake pedal reduces the air pressure and releases the brakes. Releasing the brakes lets some compressed air go out of the system, so the air pressure in the tanks is reduced. It must be made up by the air compressor. Pressing and releasing the pedal unnecessarily can let air out faster than the compressor can replace it. If the pressure gets too low, the brakes will not work.

S-cam Brakes. When you push the brake pedal, air is let into each brake chamber. Air pressure pushes the rod out, moving the slack adjuster, thus twisting the brake camshaft. This turns the S-cam (it is shaped like the letter “S”). The S-cam forces the brake shoes away from one another and presses them against the inside of the brake drum. When you release the brake pedal, the S-cam rotates back and a spring pulls the brake shoes away from the drum, letting the wheels roll freely again. See Figure 5.2.

Disc Brakes.In air-operated disc brakes, air pressure acts on a brake chamber and slack adjuster, like S-cam brakes. But instead of the S-cam, a “power screw” is used. The pressure of the brake chamber on the slack adjuster turns the power screw. The power screw clamps the disc or rotor between the brake lining pads of a caliper, similar to a large c-clamp.

All vehicles with air brakes have a pressure gauge connected to the air tank. If the vehicle has a dual air brake system, there will be a gauge for each half of the system (or a single gauge with two needles). Dual systems will be discussed later. These gauges tell you how much pressure is in the air tanks.

This gauge shows how much air pressure you are applying to the brakes. (This gauge is not on all vehicles.) Increasing application pressure to hold the same speed means the brakes are fading. You should slow down and use a lower gear. Brakes that are of adjustment, air leaks, or mechanical problems can also cause the need for increased pressure.

A low air pressure warning signal is required on vehicles with air brakes. A warning signal you can see must come on when the air pressure in the tanks falls between 55 and 75 psi (or 1/2 the compressor governor cutout pressure on older vehicles). The warning is usually a red light. A buzzer may also come on.

Drivers behind you must be warned when you put your brakes on. The air brake system does this with an electric switch that works by air pressure. The switch turns on the brake lights when you put on the air brakes.

Some vehicles made before 1975 have a front brake limiting valve and a control in the cab. The control is usually marked “normal” and “slippery.” When you put the control in the “slippery” position, the limiting valve cuts the “normal” air pressure to the front brakes by half. Limiting valves were used to reduce the chance of the front wheels skidding on slippery surfaces. However, they actually reduce the stopping power of the vehicle. Front wheel braking is good under all conditions. Tests have shown front wheel skids from braking are not likely even on ice. Make sure the control is in the “normal” position to have normal stopping power.

Many vehicles have automatic front wheel limiting valves. They reduce the air to the front brakes except when the brakes are put on very hard (60 psi or more application pressure). The driver cannot control these valves.

All trucks, truck tractors, and buses must be equipped with emergency brakes and parking brakes. They must be held on by mechanical force (because air pressure can eventually leak away). Spring brakes are usually used to meet these needs. Powerful springs are held back by air pressure when driving. If the air pressure is removed, the springs put on the brakes. A parking brake control in the cab allows the driver to let the air out of the spring brakes. This lets the springs put the brakes on. A leak in the air brake system, which causes all the air to be lost, will also cause the springs to put on the brakes.

Tractor and straight truck spring brakes will come fully on when air pressure drops to a range of 20 to 45 psi (typically 20 to 30 psi). Do not wait for the brakes to come on automatically. When the low air pressure warning light, and buzzer first come on, bring the vehicle to a safe stop right away, while you can still control the brakes.

In newer vehicles with air brakes, you put on the parking brakes using a diamond-shaped, yellow, push-pull control knob. You pull the knob out to put the parking brakes (spring brakes) on, and push it in to release them. On older vehicles, the parking brakes may be controlled by a lever. Use the parking brakes whenever you park.

Caution.Never push the brake pedal down when the spring brakes are on. If you do, the brakes could be damaged by the combined forces of the springs and the air pressure. Many brake systems are designed so this will not happen. Not all systems are set up that way, and those that are may not always work. It is much better to develop the habit of not pushing the brake pedal down when the spring brakes are on.

Modulating Control Valves. In some vehicles a control handle on the dash board may be used to apply the spring brakes gradually. This is called a modulating valve. It is spring-loaded so you have a feel for the braking action. The more you move the control lever, the harder the spring brakes come on. They work this way so you can control the spring brakes if the service brakes fail. When parking a vehicle with a modulating control valve, move the lever as far as it will go and hold it in place with the locking device.

Dual Parking Control Valves. When main air pressure is lost, the spring brakes come on. Some vehicles, such as buses, have a separate air tank which can be used to release the spring brakes. This is so you can move the vehicle in an emergency. One of the valves is a push-pull type and is used to put on the spring brakes for parking. The other valve is spring loaded in the “out” position. When you push the control in, air from the separate air tank releases the spring brakes so you can move. When you release the button, the spring brakes come on again. There is only enough air in the separate tank to do this a few times. Therefore, plan carefully when moving. Otherwise, you may be stopped in a dangerous location when the separate air supply runs out. See Figure 5.3.

Truck tractors with air brakes built on or after March 1, 1997, and other air brakes vehicles (trucks, buses, trailers, and converter dollies) built on or after March 1, 1998, are required to be equipped with anti-lock brakes. Many commercial vehicles built before these dates have been voluntarily equipped with ABS. Check the certification label for the date of manufacture to determine if your vehicle is equipped with ABS. ABS is a computerized system that keeps your wheels from locking up during hard brake applications.

Most heavy-duty vehicles use dual air brake systems for safety. A dual air brake system has 2 separate air brake systems, which use a single set of brake controls. Each system has its own air tanks, hoses, lines, etc. One system typically operates the regular brakes on the rear axle or axles. The other system operates the regular brakes on the front axle (and possibly one rear axle). Both systems supply air to the trailer (if there is one). The first system is called the “primary” system. The other is called the “secondary” system. See Figure 5.4.

Before driving a vehicle with a dual air system, allow time for the air compressor to build up a minimum of 100 psi pressure in both the primary and secondary systems. Watch the primary and secondary air pressure gauges (or needles, if the system has 2 needles in one gauge). Pay attention to the low air pressure warning light and buzzer. The warning light and buzzer should shut off when air pressure in both systems rises to a value set by the manufacturer. This value must be greater than 55 psi.

The warning light and buzzer should come on before the air pressure drops below 55 psi in either system. If this happens while driving, you should stop right away and safely park the vehicle. If one air system is very low on pressure, either the front or the rear brakes will not be operating fully. This means it will take you longer to stop. Bring the vehicle to a safe stop, and have the air brakes system fixed.

This device allows air to flow in one direction only. All air tanks on air-brake vehicles must have a check valve located between the air compressor and the first reservoir (CVC §26507). The check valve keeps air from going out if the air compressor develops a leak.

You should use the basic 7-step inspection procedure described in Section 2 to inspect your vehicle. There is more to inspect on a vehicle with air brakes than one without them. These components are discussed below, in the order that they fit into the 7-step method.

Check the air compressor drive belt (if the compressor is belt-driven). If the air compressor is belt-driven, check the condition and tightness of the belt. It should be in good condition.

The manual adjustment of an automatic adjuster to bring a brake pushrod stroke within legal limits is generally masking a mechanical problem and is not fixing it. Further, routine adjustment of most automatic adjusters will likely result in premature wear of the adjuster itself. It is recommended that when brakes equipped with automatic adjusters are found to be out of adjustment, the driver takes the vehicle to a repair facility as soon as possible to have the problem corrected. The manual adjustment of automatic slack adjusters is dangerous because it may give the driver a false sense of security regarding the effectiveness of the braking system.

Brake drums (or discs) must not have cracks longer than 1/2 the width of the friction area. Linings (friction material) must not be loose or soaked with oil or grease and must not be worn dangerously thin (less than 1/4 inch). Mechanical parts must be in place, not broken, or missing. Check the air hoses connected to the brake chambers to make sure they are not cut or worn due to rubbing.

All air brake system tests in this section are considered important and each can be considered critical parts of the in-cab air brakes tests. The items marked with an asterisk (*) in this section are required for testing purposes during the vehicle inspection portion of the CDL skills test. They may be performed in any order as long as they are performed correctly and effectively. If these items are not demonstrated and the parameters for each test are not verbalized correctly, it is considered an automatic failure of the vehicle inspection portion of the skills test.

To perform this test, the driver must start with the engine running and with the air pressure built to governor cut-out (120–140 psi or another level specified by the manufacturer). The driver identifies when cut-out occurred, shuts off the engine, chocks the wheels if necessary, releases the parking brake (all vehicles) and tractor protection valve (combination vehicle), and fully applies the foot brake. The driver then holds the foot brake for 1 minute after stabilization of the air gauge. The driver checks the air gauge to see that the air pressure drops no more than 3 pounds in one minute (single vehicle) or 4 pounds in 1 minute (combination vehicle) and listens for air leaks. The driver must identify how much air the system lost and verbalize the maximum air loss rate allowed for the representative vehicle being tested.

For a Class A combination vehicle, if the power unit is equipped with air brakes and the trailer is equipped with electric/surge brakes, the pressure drop should be no more than 3 psi.

An air loss greater than those listed above, indicates a problem in the braking system and repairs are needed before operating the vehicle. If the air loss is too much, check for air leaks and fix any that are identified.

For testing purposes, you must be able to demonstrate this test and verbalize the allowable air loss for your vehicle. For testing purposes, identify if the air loss rate is too much.

To perform this test the vehicle must have enough air pressure so the low-pressure warning signal is off. The engine maybe on or off; however, the key must be in the “on” or “battery charge” position. Next, the driver begins fanning off the air pressure by rapidly applying and releasing the foot brake. Low-air warning devices (buzzer, light, and flag) must activate before air pressure drops below 55 psi or the level specified by the manufacturer. The driver must indicate the approximate pressure when the device gave warning and identify the parameter at which this must occur; no lower than 55 psi. See Figure 5.5.

For testing purposes, identify and verbalize the pressure at which the low air pressure warning signal activates and identify the parameter(s) at which this should occur. On large buses, it is common for low-pressure warning devices to signal at 80–85 psi. If testing in a large bus, identify the parameter(s) mentioned above (55–75 psi) and inform the examiner that your vehicle’s low-pressure warning devices are designed to activate at a higher pressure.

If the warning signal does not work, you could lose air pressure and not know it. This could cause sudden emergency braking in a single-circuit air system. In dual systems, the stopping distance will be increased. Only limited braking can be done before the spring brakes come on.

To perform this test, the parking brake (all vehicles) and tractor protection valve (combination vehicles) must be released; (engine running or not) as the driver fans off the air pressure. Normally between 20-45 psi (or the level specified by the manufacturer) on a tractor-trailer combination vehicle, the tractor protection valve and parking brake valve should close (pop out). On other combination vehicle types and single vehicle types, the parking brake valve should close (pop out). The driver must identify and verbalize the approximate pressure at which the brake(s) activated.

The parking brake valve will not pop out on buses that are equipped with an emergency park brake air reservoir (tank). If your bus is equipped with an emergency park brake air tank, you must perform the spring brake test for triple reservoir vehicles to check the automatic actuation of the spring brakes.

If the parking brake valve does not pop out when the air pressure has been reduced to approximately 20 psi, you must demonstrate that the spring brakes have activated. To do this, you must:

To perform this test, the engine must be running at normal operating idle, typically 600–900 rpms. Observe the air gauge to determine if the pressure builds at the proper rate. For dual air systems, the pressure should build from approximately 85 to 100 psi within 45 seconds. For single air systems (in pre-1975 vehicles), the pressure should build from approximately 50 to 90 psi within 3 minutes.

With a basically fully-charged air system (within the effective operating range for the compressor), turn off the engine, release all brakes, and let the system settle (air gauge needle stops moving). Time for 1 minute. The air pressure should not drop more than:

Wait for normal air pressure, release the parking brake, move the vehicle forward slowly (about 5 mph), and apply the brakes firmly using the brake pedal. Note any vehicle “pulling” to one side, unusual feel, or delayed stopping action.

Stopping distance was described in Section 2.6 under “Speed and Stopping Distance.” With air brakes there is an added delay, “brake lag”. This is the time required for the brakes to work after the brake pedal is pushed. With hydraulic brakes (used on cars and light/medium trucks), the brakes work instantly. However, with air brakes, it takes a little time (one half second or more) for the air to flow through the lines to the brakes. Thus, the total stopping distance for vehicles with air brake systems is made up of 4 different factors.

The air brake lag distance at 55 mph on dry pavement adds about 32 feet. Therefore, at 55 mph for an average driver under good traction and brake conditions, the total stopping distance is over 450 feet. See Figure 5.6.

If the low air pressure warning comes on, stop and safely park your vehicle as soon as possible. There might be an air leak in the system. Controlled braking is possible only while enough air remains in the air tanks. The spring brakes will come on when the air pressure drops into the range of 20 to 45 psi. A heavily loaded vehicle will take a long distance to stop because the spring brakes do not work on all axles. Lightly loaded vehicles or vehicles on slippery roads may skid out of control when the spring brakes come on. It is much safer to stop while there is enough air in the tanks to use the foot brakes.

If your vehicle does not have automatic air tank drains, drain your air tanks at the end of each working day to remove moisture and oil. Otherwise, the brakes could fail.

What happens when you pressurize an air tank beyond its rated pressure? It fails catastrophically and ruptures in a spectacular way that you wouldn’t want to witness firsthand.

Most air compressors have a number of precautions built in to avoid the risk of a tank rupture. The compressor itself will probably have an automatic shut-off control that turns off the motor once the maximum tank pressure is reached.

In case the auto-shutoff switch fails, there will typically also be a safety valve built into the tank. Such safety valves, such as the Conrader hard seat valve shown above, are rated to specific pressures. A 100 PSI valve will open up at ~100 PSI in order to vent excess air to keep the pressure at or below 100 PSI.

Essentially, safety valves have spring-loaded pistons. Below their factory-set pressures, internal springs hold the pistons downward, creating a seal. But once the pressure inside a tank or device overcomes the built-in pressure limit of a safety valve, the piston is pushed upwards, opening the valve seal to lower and equalized the air pressure.

Safety valves typically also have loops attached to the pistons so that you can manually rapidly depressurize a device or air tank. You should check safety valves every now and then to ensure they can open freely.

There are two main types of safety valves – hard seat and soft seat valves. Hard seat valves are rugged and inexpensive, but typically leak a little bit. Soft seat valves are a little less rugged and more expensive, but they are built with better seals that aren’t as prone to leaking.

I went with Conrader hard seat valve for a recent project, but there are other good brands as well. Safety valves are available in a wide range of pressures and in different styles.

Amazon carries a couple of valves directly and through 3rd party vendors, but your best bet is to check with Grainger, Zoro, McMaster Carr, or another industrial suppliers that have wider selections.