Solenoid valves with piston, servo-assisted
A plunger-type solenoid is used as the pilot valve with this principle as well, but the main valve function is implemented by a piston. The function of this valve (e.g. Types 5404 and 6221) results from the fact that fluid is also able to flow from the supply P through a control bore in the piston or bypass channel of the main valve. If the chamber above the piston is closed off with the pilot valve de-energized, the fluid system pressure builds up there as well and the piston, which simultaneously forms the valve seal, closes the valve seat in the main valve.