Update your semi with fast and reactive rear air brakes, with the Prima Brake 4-Port 4.5 psi Relay Valve that ensures you're Ready for the Road Ahead™. 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®.