Equip your semi to be Ready for the Road Ahead , with the Prima Brake 4-Port 4.5 psi Emergency Relay Valve, a crucial element that contributes to fast and reactive air brakes. By positioning itself between the service reservoir and the rear air brakes, this replacement component is able to quickly release compressed air and actuate the air brakes. Built with a solid metal body to tolerate extreme pressures and daily use. FleetPride is your First Click in Heavy Duty .