The Bendix® TP-4™ tractor protection valve is used in the dual air brake system and houses three valves that serve the trailer and tractor braking system. The valves are: \n1. The trailer service line shut off portion of tractor protection system which operates in conjunction with the dash-mounted PP-3 trailer supply valve to manually or automatically close and retain sufficient air pressure in the towing vehicle to stop it with its service brakes. \n2. A double check valve which permits either primary or secondary circuit air pressure from the dual brake valve to actuate trailer brakes. \n3. A double check valve which permits use of a trailer control valve to control only the trailer brakes.