Principle of trailer control valve?

Working principle of trailer control valve:

Most of the handbrake systems of medium and large cars use air braking. The handbrake system of this car is usually in a constant braking state with strong springs. When the car is going to run, the driver releases the brake as a deflation action. Only when the hand brake is released, that is, before driving, the spring can be pushed away. Conventional braking is to lock the transmission shaft with a hand brake, and when the foot brakes, compressed air enters the brake chamber to lock the wheels. When the hand brake or transmission shaft fails mechanically, the hand brake fails; When any part of the air pump, pipeline, air storage tank and brake valve fails, the foot brake will fail. Dead air braking can effectively avoid these dangers. Composition and function 1) Common air brake system The common air brake system consists of brake operating mechanism, double-circuit brake mechanism, central disc brake mechanism, brake and air compressor. Brake control mechanism includes brake pedal, pedal suspension, etc. The double-loop braking mechanism includes an air storage tank, a brake valve, a low-pressure alarm, an air pressure regulator, a brake pipe, a reversing valve, a relay valve and a safety device.