11.2 Air Brake System

An air brake system performs the following basic actions:

Considerable force is available for braking because the operating pressure may be as high as 110 psi. All brakes on a vehicle, and on a trailer when one is used, are operated together by means of special regulating valves. A diagram of a typical air brake system is shown in Figure 13-26.

Figure 13-26 — Typical air brake system.


The compressor is driven from the engine crankshaft or one of the auxiliary shafts. The three common methods of driving the compressor from the engine are gear (the most common and currently used), belt (can be found on older models), and chain (the most obsolete). The compressor may be lubricated from the engine crankcase or self-lubricating. Cooling may be either by air or liquid from the engine. Compressors, having a displacement of approximately 7 cubic feet per minute (cfm), have two cylinders, while those with a displacement of 12 cfm have three cylinders.

Figure 13-27 — Typical two cylinder reciprocal air compressor.

The reciprocal air compressor operates continuously while the engine is running, but the governor controls the actual compression (Figure 13-27). The operation of the compressor is as follows:

The partial vacuum created on the piston down stroke draws air through the air strainer and intake ports into the cylinder.

As the piston starts its upstroke, the intake ports are closed off, and the air trapped in the cylinder is compressed.

The pressure developed lifts the discharge valve, and the compressed air is discharged to the reservoirs. As the piston starts its down stroke, pressure is relieved, closing the discharge valve.


The purpose of the compressor governor is to maintain the air pressure in the reservoir between the maximum pressure desired (100 to 110 psi) and the minimum pressure required automatically for safe operation (80 to 85 psi) by controlling the compressor unloading mechanism.

In the type O-1 governor, air pressure from the reservoir enters the governor through the strainer and is always present below the tower valve and in the spring tube (Figure 13-28). As the air pressure increases, the tube tends to straighten out and decrease pressure on the valve.

Figure 13-28 — Type O-1 governor.

When the reservoir air pressure reaches the cutout setting of the governor (100 to 110 psi), the spring load of the tube on the tower valve has been reduced enough to permit air pressure to raise the tower valve off its seat. This movement of the lower valve raises the upper valve to its seat, which closes the exhaust port. Air then flows up through the small hole in the lower valve and out the upper connection to the unloader assembly located in the compressor cylinder head. When the unloader valves open, the compression of air is stopped.

When reservoir pressure is reduced to the cut-in setting of the compressor governor (80 to 85 psi), the governor tube again exerts sufficient spring pressure on the valve mechanism to depress and close the lower valve and open the upper valve, thereby shutting off and exhausting the air from the compressor unloading mechanism, and compression is resumed.

The pressure range and setting should be checked periodically using an air gauge known to be accurate. Pressure range may be changed in the type O-1 governor by adding shims beneath the upper valve guide to decrease the range, or removing shims to increase the range. Pressure settings may be changed, if necessary, by turning the adjusting screw to the left to increase the setting or to the right to decrease the setting.

The strainer should be removed periodically and cleaned. Check the governor periodically for excessive leakage in both the cut-in and cut-out positions. If the governor fails to operate properly, it should be repaired or replaced.

In the type D governor, when the reservoir pressure reaches the cut-out setting (100 to 110 psi), the governor diaphragm is subjected to sufficient air pressure to overcome the spring loading (Figure 13-29. This action allows the valve mechanism to move up, permitting the exhaust stem to close the exhaust valve and to open the inlet valve.

Figure 13-29 — Type D governor.

Reservoir pressure then passes through the governor to operate the compressor unloading mechanism, stopping further compression of the air compressor.

When the reservoir pressure is reduced to the cut-in setting (80 to 85 psi), the spring loading within the governor overcomes the air pressure under the diaphragm. The valve mechanism is actuated, closing the inlet valve and opening the exhaust valve, thereby shutting off and exhausting the air from the compressor unloading mechanism, and compression is resumed.

Pressure range and setting should be checked periodically, using an accurate air gauge. The pressure range (pressure differential) between loading and unloading of the type D governor is a function of the design of the unit and should not be changed. The designed range for this governor is approximately 20 percent of the cut-out pressure setting. The pressure settings of the type D governor may be adjusted by turning the adjusting nut clockwise to increase or counterclockwise to decrease the settings.

Both strainers should be removed periodically and cleaned or replaced. The governor should periodically be checked for leakage at the exhaust port in both the cut- in and cutout positions. If the governor fails to operate properly, it should be repaired or replaced.

Unloader Assembly

The unloader assembly is mounted in the compressor head and controlled by the governor (Figure 13-30). The unloader valve may be either a poppet-type or a spring-loaded control valve. Air pressure from the governor opens the unloader valves to unload or stop compression in the compressor.

Figure 13-30 — Unloader assembly.

When the reservoir air pressure reaches the maximum setting of the governor, air under pressure is allowed by the governor to pass into a cavity below an unloading diaphragm. This air pressure lifts one end of the unloading lever, which pivots on its pin and forces the unloading valves off their seats. With the unloading valves off their seats, the unloading cavity forms a passage between the cylinders above the pistons. Air then passes back and forth through the cavity between the cylinders and compression is stopped. A drop in air pressure below the minimum setting of the governor causes it to release the air pressure from beneath the unloading diaphragm, allowing the unloading valves to return to their seats resuming compression.

Air Tanks (Reservoirs)

The two steel air tanks, commonly known as reservoirs, are used to cool, store, remove moisture from the air, and give a smooth flow of air to the brake system.

At the bottom of each tank is a drain valve (Figure 13-31). This valve is used to allow the operator a means to drain the air from the tanks daily, thereby preventing any moisture buildup in the system. Moisture in the system prevents the brakes from actuating smoothly. A safety valve is located on top of the first reservoir and consists of an adjustable spring-loaded bail-check valve in a body. It is used to protect the system against excessive pressures, normally set at approximately 150 psi.

Figure 13-31 — Air reservoir with an air drain valve.

Brake Chambers

The brake chamber converts the energy of the compressed air into mechanical force to operate the brakes (Figure 13-32). When the brake pedal is actuated, air under pressure enters the brake chamber behind the diaphragm and forces the pushrod out against the return spring force. Because the yoke on the end of the pushrod is connected to the slack adjuster, this movement rotates the slack adjuster, brake camshaft, and cam to apply the brakes.

Figure 13-32 — Air brake chamber.

When the pedal is released, air is forced from the brake chamber by the brake shoe return spring acting on the linkage. After the shoes reach the fully released position, the return springs acting on the diaphragm causes it to return to its original position in the chamber.

When performing maintenance of the brake system, check the brake chamber alignment to avoid binding action. Check the pushrod travel periodically, and when necessary,

adjust the brakes so that pushrod travel is as short as possible without the brakes dragging. The pushrod length should be adjusted so that the angle between the center line of the slack adjuster and the brake chamber pushrod is 90 degrees when the pushrod is extended to the center of its working stroke.

Replace the diaphragm if it is worn or leaking. Replace the boot if it is worn or cracked. With the brakes applied, cover the edges of the diaphragm and bolt with soapy water to detect leakage. If leaks are present, tighten the bolts uniformly until the leaks stop. Bolts should not be tightened so that the diaphragm shows signs of bulging or distortion.

Slack Adjusters

The slack adjusters function as adjustable levers and provide a means of adjusting the brakes to compensate for wear of linings (Figure 13-33). Air pressure, admitted to the brake chamber when the brake pedal is depressed, moves the slack adjuster toward the position indicated by the dotted lines. Adjustments are made to ensure the travel does not exceed 1 inch.

Figure 13-33 — Slack adjuster.

The entire slack adjuster rotates as a lever with the brake camshaft as the brakes are applied or released. Turning the adjusting screw makes the brake adjustments necessary to maintain proper slack adjuster arm travel (shoe and drum clearance). This action rotates the worm gear, camshaft, and cam, expanding the brake shoes so that the slack caused by brake lining wear is eliminated and the slack adjuster arm travel is returned to the correct setting. The movement of the cam forces the brake shoes against the brake drum. Friction of the brake lining against the drum stops the turning movement of the wheel. When the brakes are released, the brake shoe return spring pulls the shoes back to a DISENGAGED position.

Brake Valves

Numerous brake valves are used in an air brake system. These valves either apply or release air from the brakes and work together to ensure control and safe braking application. These valves are as follows:

In the following paragraphs we will discuss each valve in more detail.

Treadle Valve

The treadle valve controls the air pressure delivered to the brake chambers. When the treadle valve is depressed, force is transmitted to the pressure-regulating spring and diaphragm that are moved downward and contact the exhaust valve and close it.

Continued movement opens the inlet valve and air pressure from the reservoir flows through the valve and into the delivery lines to apply the brakes. As the air pressure increases below the diaphragm, it overcomes the force above the diaphragm and the diaphragm rises slightly. This action allows the inlet valve to close but also keeps the exhaust valve closed, thereby obtaining a balanced position. Further depression of the treadle valve increases the forces above the diaphragm and correspondingly increases the delivered air pressure until a new balanced position is reached.

Maintenance of the treadle valve consists of periodic lubrication of the hinge and roller. Should the valve malfunction, it can be disassembled and cleaned. After cleaning, the internal parts should be lubricated with Vaseline before reassembly. This prevents moisture in the air system from causing corrosion and freezing of the valve. If cleaning does not remedy the malfunction, the valve must be replaced.

Trailer Control Valve

The independent trailer control valve provides the operator with control of the trailing load at all times (Figure 13-34). This valve functions in the same manner as the treadle valve except that the handle is turned, rather than depressed, to operate the valve.

Figure 13-34 — Trailer control valve.

Quick-Release Valve

The quick-release valve exhausts brake chamber air pressure and speeds up brake release by reducing the distance the air would have to travel back to the brake valve exhaust port (Figure 13-35).

Figure 13-35 — Quick-release valve.

When the brakes are engaged, air from the brake valve enters into the quick-release valve, forcing the diaphragm down and closing off the exhaust port. This action allows air pressure to rush through the quick-release valve outlet ports to the wheel brake chambers. When the brakes are released, the air pressure above the quick-release diaphragm is exhausted at the brake valve. As air pressure above the diaphragm is released, the air pressure below the diaphragm raises off the exhaust port. This action allows the air in the brake chambers to exhaust at the quick-release valve.

When air is leaking from the system, a leakage test can determine if there is air leaking at the quick-release valve. To perform the leakage test, apply the brakes and coat the exhaust port with soapsuds. If air bubbles form, this is a sign of a defective valve, which can be corrected either by cleaning and replacing worn parts or by replacing the unit. Dirt, a worn diaphragm, or a worn seat causes leakage.

Combined-Limiting and Quick-Release Valve

The combined-limiting and quick-release valve is used in combination with a two-way check valve in the air brake system of trucks and tractors. The combined-limiting and quick-release valve is interchangeable in mounting with the quick-release valve and serves the same purpose with the additional function of providing an automatic reduction of front-wheel brake pressure, at the option of the operator, on slippery roads.

Tractor Protection Valve

The primary purpose of the tractor protection valve is to protect the tractor air brake system under trailer breakaway conditions and under conditions where severe leakage develops in the tractor or trailer (Figure 13-36).

Figure 13-36 — Tractor protection valve and switch.

The tractor protection system functions as a set of remotely controlled cutout valves (Figure 13-37). The trailer service and emergency lines pass through the valve. When the control valve is in the NORMAL position, service and emergency braking functions of both the tractor and trailer are normal. When the valve lever is in the EMERGENCY position, the trailer air brakes lines are closed off.

Figure 13-37 — Tractor protection valve piping.

Should a condition resulting in severe air loss from the tractor or trailer air brake system be detected or if for any other reason it is desirable to cause an emergency application of the trailer brakes, the operator can move the control valve lever to the EMERGENCY position. At this time both the trailer service and emergency brake line will be closed off at the tractor protection valve. Such operation offers a convenient daily check of the relay emergency valve on the trailer where tractors and trailers are not disconnected over long periods of time. The operator should move the control to the EMERGENCY position when disconnecting a trailer or when operating a tractor without a trailer if cut- off valves are not installed in the trailer connections on the tractor. The tractor protection valve should NOT be used as a parking brake because it was not designed for that purpose.

Relay Emergency Valve

The relay emergency valve (Figure 13-38) acts as a relay station to speed up the application and release of trailer brakes (Figure 13-38). It automatically applies the trailer brakes when the emergency line of the trailer is broken, disconnected, or otherwise vented to the atmosphere if the trailer air brake system is charged. It is used on trailers that require an emergency brake application upon breakaway from the truck or tractor.

Figure 13-38 — Relay emergency valve.

When a tractor is connected to a trailer and the service and emergency lines are opened, the relay emergency valve permits charging the trailer air brake reservoir to approximately the same air pressure as that in the tractor reservoirs. During normal operation of a tractor-trailer unit, the relay emergency valve functions as a relay valve and synchronizes trailer service brake air pressure and tractor service brake air pressure as the treadle valve on the tractor is operated. The trailer brakes can also be applied independently of the tractor brakes by use of the hand control on the tractor protection valve on the tractor and the relay emergency valve on the trailer.

If a trailer is disconnected from a tractor for loading or unloading, if the trailer is separated from the tractor under emergency breakaway conditions, or if the emergency line of the trailer is vented to the atmosphere by other means, the relay emergency valve applies the trailer brakes. This is automatically achieved by using the existing trailer reservoir air pressure. If the trailer is to remain parked under these conditions, the wheels should be blocked to prevent the possibility of a runaway.

If you are required to release the emergency brake application on a trailer under these conditions, the trailer reservoir drain valve can be opened or the trailer air brake system can be recharged through the trailer emergency line.

You can check the relay emergency valve by moving the tractor protection valve control lever to the EMERGENCY position, if tractor protection equipment is installed. If no tractor protection is installed, you can check the valve by closing the emergency line cut-out valve and uncoupling the emergency brake line. Either way the trailer brakes should apply automatically. Trailer brakes should release, in the first case, when the tractor protection valve control lever is moved to the NORMAL position, and in the second case, when the emergency line is coupled and the cutout valve is opened.

You can check the relay emergency valve for leakage by applying soapsuds with the brakes released. Check the emergency air line coupling with soapsuds to determine leakage with the valve in emergency application position. Leakage may be caused by dirt or worn parts, which may be corrected by cleaning and/or replacing the unit.

Check Valves

Check valves are located in the lines of air brake systems to prevent the loss of air should the line rupture while in operation. These are placed at the entrance of the main air tanks and prevent the loss of air

should the inlet line from the compressor fail. The ball-type check valve is typical of the type used on trailer braking systems (Figure 13-39). Check valves may be either disc or ball and double or single units. Regardless of their design, their function is the same.

Figure 13-39 — Ball-type single check valve.

Air Hoses and Fittings

Air hoses and fittings provide a means of making a flexible air connection between points on a vehicle which change their position in relation to each other or between two vehicles (Figure 13-40). All air brake assemblies used to connect the air brake systems from one vehicle to another are equipped with detachable fittings and spring guards.

Figure 13-40 — Air hose and fittings.

When installing a hose assembly where both ends are permanently connected, use the air hose connector assembly at each end as the union to permit tightening the hose connectors in place. Loosen the nut on one of the connector assemblies and then turn the hose in the loose connector to avoid kinking the hose.

To prevent dirt and moisture from entering unused air lines, use dummy couplings (Figure 13-41). The two types of dummy couplings are as follows:

Figure 13-41 — Dummy couplings.

System Switches and Indicators

The switches and indicators in an air brake system are designed as safety devices. The two most common safety devices found in an air brake system are the low-pressure warning indicator and the stoplight switch.

Low-Pressure Warning Indicator

The low-pressure warning indicator is an electro-pneumatic switch connected with a warning buzzer and, in some designs, a warning light or both (Figure 13-42). It remains in the OPEN position when air pressure is above approximately 60 psi. When pressure drops below 60 psi, the spring forces the diaphragm down and closes the contacts, which operate the warning device. Normal operating pressure is 60 psi, plus or minus 6 pounds.

Figure 13-42 — Low-pressure warning indicator.

Stoplight Switches

Stoplight switches in an air brake system are electro-pneumatic devices which operate in conjunction with the treadle valve to close the stoplight circuit when the brakes are applied (Figure 13-43). When air pressure from the treadle valve enters the cavity on the one side of the diaphragm, the diaphragm changes position. This action overcomes the force of the spring and moves the contact plunger until the contacts close. This closes the stoplight electrical circuit, causing the brake lights to come on. The switch is designed to close as soon as 5 psi is delivered to it. This means that the stoplight circuit closes immediately on brake application.

Figure 13-43 — Stoplight switch.

Servicing Air Brakes

Servicing is the most important part of air brake maintenance. If the air brake system is kept clean, tight, and moisture-free, brake failures will be few and far between.

Particular care must be taken to keep the air compressor intake filters clean and foreign material out of the lines.

The basic test made to an air brake system is the operational test. This test may be performed on the road or in the shop. During an operational test, the brakes are applied and released while observing for equal application, sluggish engagement or release, binding linkage, and exhaust of units.

To check the leakage of the overall system, fully charge the system, shut off the ignition, and observe the pressure drop on the gauge mounted on the vehicle dash. The maximum leakage will be expressed in pounds per a specific time.



Before making any leakage or pressure test, consult the manufacturer’s specifications for correct pressure and maximum leakage.

To determine if leakage of various components is within permissible or authorized limits, use the soapsuds test. To make this test, use a thick mixture of soapsuds, but do not use lye soap. Apply this mixture to places in the system where leakage may occur.

While some places are authorized some amount of leakage, others are not. For example, castings and the tube in the governor should have no leakage. Points with authorized leakage will have a specified maximum in pounds per a specified time.

Soapsuds can also be used to check the internal condition of a component. By covering exhaust ports or casting openings, you can check the condition of the diaphragms and valves. For example, to check the condition of the treadle valve, release the brakes and cover the exhaust ports with soapsuds. Engage the brakes; any leakage indicates the valve is not sealing properly. If the diaphragm in the brake chamber is faulty, leakage will appear around the pushrod with the brakes applied.

As with the drum brake system, the linings used with air brakes gradually wear from use and require periodic adjustment or replacement. Always consult the manufacturer’s specifications before making any adjustments to the air brake system. This is to ensure that the correct adjustment is made and that any variations in procedure are followed.


A typical air ABS system requires the following components:

Wheel speed sensors are electromagnetic devices used to signal wheel speed information to the ABS module. The sensor consists of a toothed ring called a reluctor wheel or tone ring, and a permanent magnet or wheel end sensor (Figure 13-44). This system is called a pulse generator. It produces an AC signal and shares the principles with the shaft sensors on the chassis including the road speed sensor and engine rpm sensor.

Figure 13-44 — Wheel speed sensor.

The voltage and frequency of this AC signal will rise in exact proportion to wheel speed. This signal is sent to the ABS module, which converts the pulse to an actual wheel speed so it can be used to manage the air pressure delivered to the brake chambers it controls.

An ABS module can be called the system controller or an electronic control unit (ECU). It is a simple computer that receives input signals and continuously monitors input wheel speed. When wheel lock up is detected, the ECU outputs an electrical signal to the solenoids in the modulator to exhaust air pressure routed to the service brake chamber. This momentarily relieves the service application pressure on the brake chambers and releases the brake momentarily to reduce lock up (Figure 13-45).

Figure 13-45 — Air brake antilock brake system.

When the operator lets off the brake pedal or when the vehicle comes to a stop, the air brake system resumes under normal operation.

Test Your Knowledge

2. (True or False) The function of the governor in an air brake system is to maintain the air pressure in the reservoir.

A. True
B. False


You learned about the systems involved in controlling and repairing brakes, and you have a clear understanding of how important it is that they function properly. This knowledge will enable you to be a better construction mechanic as you provide safe transportation by servicing and repairing hydraulic and air brake systems.