Product Details: Prepare your semi to be Ready for the Road Ahead™, with the Bendix R-12® 4 psi Air Brake Relay Valve that contributes to fast and reactive air brakes. Due to its placement between the service reservoir and the rear air brakes, this replacement component is able to quickly release compressed air to actuate the air brakes. Constructed with a rugged metal body to endure extreme pressures and regular use. FleetPride is your First Click in Heavy Duty®.