Air Brake System

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An air brake system uses compressed air to apply the brakes. Air under pressure can be conveniently stored and carried through lines or tubes. Considerable force is available for braking since operating air pressure may be as high as 100 psi. All brakes on a vehicle and on a trailer (when one is used) are operated together by a brake valve. This valve and the relative location of most of the basic assemblies of an air brake system are shown in figure 3-44.

Air Compressor

The air compressor pumps air into the air storage tanks (reservoirs). The air compressor is driven by the engine through gears or a V-belt. The compressor may be air-cooled or may be cooled by the engine lubrication system. It may have its own oil supply or be lubricated by engine oil. If the compressor has its own oil supply, the oil should be checked during your prestart operations.


The governor controls the air compressor output. When air tank pressures rise to the cutout level at about

Figure 3-44.-Typical air brake system.

125 pounds per square inch (psi), the governor stops the compressor from pumping air. When the tank pressure falls to the cut-in pressure at about 100 psi, the governor allows the compressor to start pumping again.

Air Storage Tanks

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

NOTE: Compressed air usually has some water and some compressor oil in it which is bad for the air brake system. For example, 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; therefore, each air tank is equipped with a drain valve in the bottom.

The two types of drain valves areas follows:

1. The manual valve shown in figure 3-45 is operated by turning it a quarter turn or by pulling a cable. Part of your post-operational procedures is to drain all air tanks at the end of each day.

2. The automatic valve automatically expels the water and oil. This system may also be equipped with a manual drain.

Alcohol Evaporator

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

Figure 3-45.-Air tank manual drain valve.

If your vehicle has an alcohol system, the container should be checked during each prestart operation and filled up as necessary. Daily draining of the air tanks is still required to get rid of the water and oil.

Safety Valve

A safety relief valve is installed in the first tank into which the air compressor pumps air. 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 has to release air pressure, something is wrong in the air brake system. This should be documented to inform the mechanic inspectors.

Brake Pedal

The brakes are applied by depressing the brake pedal (also called the foot valve, or treadle valve) that gives the operator control of the air brake system. When the brake pedal is engaged, air from the air tanks flows through the brake pedal valve through the brake lines to the brake chambers close to the wheel brakes that contain flexible diaphragms. The force of the air admitted into these chambers causes the diaphragms to operate the brake shoes through a mechanical linkage.

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 allows some compressed air out of the system; therefore, the air pressure in the tanks is reduced and it must be recharged by the air compressor. Pressing and releasing the pedal unnecessarily may release air out faster than the compressor can replace it, and should the pressure become too low, the brakes cannot work properly and brake failure will occur.

Pressure Gauge

An air pressure gauge lets you know if you have proper air pressure within the reservoir. A low air warning device should cut on before the pressure drops to less than 60 psi in the air tank. This gauge is usually on the instrument panel of a truck or bus. If the pressure fails to buildup or exceeds the maximum limits after building up, secure the truck until the fault is corrected.

Hand Brake Valve

Independent control of brakes is necessary under bad conditions, especially if you have to put on the

Figure 3-46.-Hand brake valve.

trailer brakes without applying the truck or tractor brakes. The hand brake valve or independent trailer control valve, as shown in figures 3-44 and 3-46, provides the operator control of the trailing load at all times.

Figure 3-48.-Air-over-hydraulic power cylinder.

NOTE: More information about the air brake system is in the chapter covering tractor and trailer operations.


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