The pilot-operated check valve allows the fluid to flow from 1 to 2 if there is a sufficient pressure differential between the port 1 and 2. In general the differential pressure between these ports needs to be higher than the cracking pressure but the exact pressure required also depends on the mode of operation defined in the "Pilot Relation" parameter and on the "Control Ratio" parameter. The fluid can flow from port 2 to port 1 when the pressure at 3 multiplied by the "Pilot Ratio" is greater than the pressure at 2 plus the "Cracking Pressure" of the valve. Port 3 is a pilot port and does not allow any fluid to flow, only the pressure is transmitted. For the valve with drain, port 4 is a drain port that makes it possible to simulate a leakage flow by adding a component failure. For more details, see the "Data" section below.