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 exception to this is the Bendix+ PP'8 valve and some Bendix+ PP'1+ valves which do not contain a spring The PP'8 valve also has a larger diameter shaft for button mounting so that when installed on the same panel with other push-pull valves the buttons cannot be inadvertently mixed The PP-8 valve is normally used to operate tractor spring brakes independently from the trailer