The pressure protection valve is located on the top of the dual compartment air reservoir.

This valve protects the air supply to the front brakes if a failure occurs in the rear brake compartment of the air reservoir, or if a failure occurs in the delivery side of the rear brake system. The valve has a supply port and a delivery port.

The valve allows air to flow from the supply port to the delivery port as long as at least 620 kPa (90 psi) is available. If the air pressure decreases below the specified closing pressure, the internal regulating spring moves the piston closing the inlet valve. The remaining air pressure at the front brake system side is retained.