The Bendix push-pull -PP- control valves are manually operated on'off air control valves with an exhaust function Most are pressure sensitive, so that they will automatically move from the applied to the exhaust position as supply pressure is reduced to a certain minimum, depending on the spring installed The Bendix+ PP'2 valve has an auxiliary port which may be plumbed into a service brake line to release the spring brakes if a service application is made, preventing compounding of forces on the foundation brakes