Air brake (road vehicle)

In the early 20th century, after its advantages were proven in railway use, it was adopted by manufacturers of trucks and heavy road vehicles.

The parking brakes use a disc or drum arrangement which is designed to be held in the 'applied' position by spring pressure.

System pressure is regulated by a governor between a minimum and maximum value;[5] the governor unloads the compressor when the maximum system pressure has been achieved and loads the compressor when it falls below its minimum setpoint.

[4][6] When the compressor is loaded, compressed air is first routed through a cooling coil and into an air dryer (3) which removes moisture and oil impurities and also may include a pressure regulator (2), safety valve and smaller purge reservoir (4).

As an alternative to the air dryer, the supply system can be equipped with an anti-freeze device and oil separator.

The compressed air is then stored in a supply reservoir (6); the supply reservoir, which is physically closest to the compressor, is also called a wet tank because the majority of oil and water from the compressor gather here.

The supply system also includes various check,[8] pressure limiting, drain and safety valves.

The dual-service brake circuits are further split into front- and rear-wheel circuits which receive compressed air from their individual supply reservoirs for added safety in case of an air leak.

Repeated application of the service brakes will deplete the air reservoir pressure, prompting the governor to load the compressor again.

The supply line receives air from the prime mover park brake air tank via a park brake relay valve and the control line is regulated via the trailer-brake relay valve.

The operating signals for the relay are provided by the prime mover brake-pedal air valve, trailer service brake hand control (subject to local heavy vehicle legislation) and the prime mover park brake hand control.