Product Details: Ensure your semi is ready for the journey ahead with fast and reactive air brakes, with the OTR 4-Port 4 psi Relay Valve with Expertise Built In™. To quickly apply compressed air to the rear air brakes, this replacement component acts as a gateway between the service reservoir and the air brakes. Fashioned with a durable metal body to tolerate extreme pressures and regular use. FleetPride is your First Click in Heavy Duty®.