Product Details: Update your semi with fast and reactive rear air brakes, with the Bendix R-12® 5.5 psi Air Brake Relay Valve that ensures you're Ready for the Road Ahead™. 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®.