The operation of air brakes is similar to the hydraulic brakes except that in their case compressed air is used to apply brakes instead of hydraulic pressure. Air brakes are commonly used on heavy vehicles like trucks, buses, etc.

The complete layout circuit is shown in figure. The compressor takes air from the atmosphere through the filter and the compressed air is sent to the Reserve through the unloader valve, which gets lifted at a predetermined reserve oil pressure (about 900 kilo Pascal) and relieves the compressor of load. From the Reservoir the air goes to various accessories and also to the break Chambers also called the diaphragm units at each wheel through the brake valve. The control of brake valve is with the driver who can control the intensity of braking according to the requirements. When the brakes are applied, the air pressure in the Reservoir decreases. When the pressure drops to approximately 700 kilo Pascal, governor again cuts in the compressor to raise system pressure. In case the air system pressure falls to about 400KPA, a warning usually in the form of a buzzer is sounded.