Check the listing for details. Single Port Air Compressor Pressure Switch Control Valve 145-175 PSI w/ Unloader. Condition: New. Listed at 17.95 USD. LEFOO 140-175 PSI air compressor pressure switch control valve with a single female NPT air inlet connection port. The switch listed is a universal replacement pressure switch that replaces brands such as Furnas, Square-D and more. This switch is equipped with an unloader valve, on / off lever, and a single 1/4" female NPT air inlet port. ( Note: NPT measures about 3/4" in diameter.) The unloader equipped on this switch is used to unload the air pressure off the head of the compressor when the compressor shuts off, this way when it starts there is no pressure on the piston and the compressor is able to start load free. This valve usually has a 1/4" plastic or copper line that runs to the check valve or the head of the compressor.