The correct answer is: The one way check valve. The one way check valve sits between the air compressor and the main air tanks. It allows air flow in one direction only, which prevents leaks in the air compressor from draining the air tanks. The trailer protection valve's purpose is to keep air in the brake system in case the trailer breaks away or develops a bad leak. A safety relief valve is installed in the first tank that the air compressor pumps air into. A safety valve protects the tank and system from too much pressure by opening up at 150 psi. The treadle valve is another name for the foot pedal.