Get your semi Ready for the Road Ahead with fast and reactive rear air brakes, with the Sealco 4-Port 4.5 psi Emergency 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 .