Unlike liquids, gases are compressed easily. If a gas, such as air, is contained and a force applied to it, it is compressed and has less volume. Placing a weight on a piston that fits in the container can exert such a force. The air that originally filled the entire container is pressed into only a portion of the container due to the force of the weight upon it. The pressure of the compressed air, resulting from the force exerted upon it by the weight, will be distributed equally in all directions just as it is in a liquid.

An air brake system performs the following basic actions:

An air pump or compressor driven by the engine is used to compress air and force it into a reservoir where it is forced under pressure and made available for operating the brakes.

Air under pressure in the reservoir is released to the brake lines by an air valve operated by the brake pedal.

This released air goes to brake chambers (located at each wheel) that contains a flexible diaphragm. Against this diaphragm is a plate that is connected directly to the mechanism on the wheel brakes by linkage.

The force of the compressed air admitted to the brake chamber causes the diaphragm to move the plate and operate the brake shoes through the linkage.

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 7-39.