<P> The air compressor is driven by the engine either by crankshaft pulley via a belt or directly from the engine timing gears . It is lubricated and cooled by the engine lubrication and cooling systems . Compressed air is first routed through a cooling coil and into an air dryer which removes moisture and oil impurities and also may include a pressure regulator, safety valve and smaller purge reservoir . 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 (also called a wet tank) from which it is then distributed via a four way protection valve into the primary reservoir (rear brake reservoir) and the secondary reservoir (front / trailer brake reservoir), a parking brake reservoir and an auxiliary air supply distribution point . The system also includes various check, pressure limiting, drain and safety valves . </P> <P> Air brake systems may include a wig wag device which deploys to warn the driver if the system air pressure drops too low . </P> <P> The control system is further divided into two service brake circuits, the parking brake circuit, and the trailer brake circuit . The dual service brake circuits are further split into front and rear wheel circuits which receive compressed air from their individual reservoirs for added safety in case of an air leak . The service brakes are applied by means of a brake pedal air valve which regulates both circuits . The parking brake is the air operated spring brake type where its applied by spring force in the spring brake cylinder and released by compressed air via hand control valve . The trailer brake consists of a direct two line system: the supply line (marked red) and the separate control or service line (marked blue). 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 . </P> <Ul> <Li> <P> Trailer Control Valve </P> </Li> <Li> <P> Spring brake cylinder </P> </Li> <Li> <P> Air brake foot valve </P> </Li> <Li> <P> Trailer brake relay valve </P> </Li> <Li> <P> Truck air compressor </P> </Li> <Li> <P> Air dryer </P> </Li> <Li> <P> Air brake relay valve </P> </Li> <Li> <P> Four way protection valve </P> </Li> </Ul>

What is the basic principle of the dual air brake system
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