Abstract: A relay valve for use in vehicle air brake systems has a housing with a service port, supply port, delivery ports and an exhaust port. A piston and shuttle are movable within the housing to control communication between the ports. The shuttle carries a seal member which, in the closed position, is in sealing relation with both the piston and the housing, thereby closing communication between the supply and delivery ports and between the delivery and exhaust ports. There are means to vent pressure buildup behind the shuttle seal thereby preventing accidental blowout of the seal.