Supply System: The heart of any air brake system is the compressor, Compressor is the device which generates and in a way regulates the flow of compressed air in the system. The compressor is powered by the engine directly and uses the common lubricant available with the engine.
Compressed air is pushed through a cooling coil and into an air dryer. From here the air is stored in a reservoir tank for use. Reservoir tank is connected with an intricate network of circuits for front brakes, rear brakes, and the parking brakes. The supply system also contains the drain valve, pressure limiting valve, and the safety valve.
Control System: Control system consists broadly of service break circuit, parking brake circuit and the trailer brake circuit (if applicable). Service break circuit consists of two individual break circuits each for front and the rear brake. Both these circuits are connected to their special reservoirs for added safety in case the master reservoir fails.
Parking break circuit is connected to a spring mechanism in which the air pressure is used to keep the spring in unlocked position. A drop in pressure of this circuit results in engaging of parking brakes. The trailer brake system has its own lines for operation and is used when there is a trailer attached to the vehicle. It has a supply line and a control line. The supply line is fed by the master reservoir and control line gets its signal from the service break system for better braking.
Thats the basic crux of an air brake system.