Prepare your semi to be Ready for the Road Ahead , with the Prima Brake R-12 Type 4 psi Relay Valve that contributes to fast and reactive air brakes. With a fixed location between the rear air brakes and the service reservoir, this replacement component allows for the immediate application of compressed air to the brakes. Manufactured with a solid metal body to endure extreme pressures and consistent use. FleetPride is your First Click in Heavy Duty .