<P> An air brake or, more formally, a compressed air brake system, is a type of friction brake for vehicles in which compressed air pressing on a piston is used to apply the pressure to the brake pad needed to stop the vehicle . Air brakes are used in large heavy vehicles, particularly those having multiple trailers which must be linked into the brake system, such as trucks, buses, trailers, and semi-trailers, in addition to their use in railroad trains . George Westinghouse first developed air brakes for use in railway service . He patented a safer air brake on March 5, 1872 . Westinghouse made numerous alterations to improve his air pressured brake invention, which led to various forms of the automatic brake . In the early 20th century, after its advantages were proven in railway use, it was adopted by manufacturers of trucks and heavy road vehicles . </P> <P> Air brake systems are typically used on heavy trucks and buses . The system consists of service brakes, parking brakes, a control pedal, and an air storage tank . For the parking brake, there's a disc or drum brake arrangement which is designed to be held in the' applied' position by spring pressure . Air pressure must be produced to release these "spring brake" parking brakes . For the service brakes (the ones used while driving for slowing or stopping) to be applied, the brake pedal is pushed, routing the air under pressure (approx 100--120 psi or 690--830 kPa or 6.89--8.27 bar) to the brake chamber, causing the brake to be engaged . Most types of truck air brakes are drum brakes, though there is an increasing trend towards the use of disc brakes in this application . The air compressor draws filtered air from the atmosphere and forces it into high - pressure reservoirs at around 120 psi (830 kPa; 8.3 bar). Most heavy vehicles have a gauge within the driver's view, indicating the availability of air pressure for safe vehicle operation, often including warning tones or lights . A mechanical "wig wag" that automatically drops down into the driver's field of vision when the pressure drops below a certain point is also common . Setting of the parking / emergency brake releases the pressurized air in the lines between the compressed air storage tank and the brakes, thus allowing the spring actuated parking brake to engage . A sudden loss of air pressure would result in full spring brake pressure immediately . </P>

When were air brakes first used on trucks