Pilot operated check valves

Pilot operated check valves work by allowing free flow from the inlet port through the outlet port. Supplying a pilot pressure to the pilot port allows flow in the opposite direction. 

Pilot operated check valves (PO Check Valves) are typically used to hold pressure in a cylinder in the event of a drop in pressure or a total loss of pressure. The pneumatic check valve locks air in the device until the pilot signal is energized, at which time the valve opens and air evacuates the cylinder.