The Bendix® PP-3™ push-pull trailer supply valve, installed in the cab, operates in conjunction with a TP-3™ tractor protection valve to supply air under pressure to the trailer and protect the tractor air supply in case of a break-away or an air leak on the trailer which would reduce the pressure in the trailer supply line below the design trip point of the PP-3™ valve. The supply port of the PP-3™ valve is supplied from the towing vehicle reservoir. Its delivery goes directly to the supply port on the TP-3™ tractor protection valve.