A pilot operated checkvalve is used to keep a part of the system free from internal leakage for example a hydraulic cilinder or motor. A very good example is the application of the pilot operated checkvalve
on the cilinder of the outrigger legs of a crane. The cilinder is connected to port B of the checkvalve. When oil is supplied to port A the oil can flow freely towards port B and to the cilinder. When the leg has to be retracted oil is supplied to te rodside of the cilinder. The pressure at the rodside of the cilinder is used as pilot pressure on port Z for opening the checkvalve. Now the oil can flow back from port B to port A. The pressure at port Z needed to open the checkvalve against the cilinderpressure behind the main valve is about 1/3 to 1/10 (called the opening ratio) of the cilinderpressure.