Outfit your semi to be Ready for the Road Ahead with fast and reactive rear air brakes, with the Wabco 12V 5.5 psi Air Brake Relay Valve. This replacement component allows for the immediate application of compressed air to the rear air brakes, by acting as a gateway between the service reservoir and the brakes. Engineered with a durable metal body to withstand extreme pressures and consistent use. FleetPride is your First Click in Heavy Duty .