Must be used whenever two sources of air are used to operate a single device or system.
For CDL air brake test, the driver must know there are 2 in the system. On located between the brake pedal and the hand valve. And the second is located between the primary and secondary air tanks.
Common examples are:
1. Allows control of the trailer service brakes by use of the hand valve or foot valve.
- If both are applied at the same time, the trailer service brakes will apply at the higher application. So if the hand valve is applied at 60psi (410kPa) and the brake pedal at 20psi (130kPa), the trailer brakes will apply at 60psi.
2. Allows blended supply air pressure taken from the primary and secondary reservoirs.
- If one of the primary subsystem loses air pressure, the two-way check valve will redirect air from the higher pressured secondary system and deliver it to the rear spring brake chambers so that the spring brakes do not apply automatically. In this way, the driver continues to have the emergency spring brakes released and the secondary service brakes working. In this way, she can bring the vehicle to a safe stop.
3. Can be used as a simple anti-compounding device for spring parking brakes.