They are also used as prefill or dump valves for large hydraulic presses or cylinders. A small integral de-compression valve can be specified to bleed off the high-pressure before the main valve opens. This can help reduce the shock of opening a large valve under high-pressure.
The operation of this valve is very similar to a standard spring loaded check valve, but with one important difference: The pilot cylinder can, when actuated, hold the valve open so that fluid may flow freely in either direction. When the pilot pressure is removed, the flow in one direction is stopped just as in a check valve.
