The pilot-operated check valve is shown in Fig. 7.11. For flow in the forward direction, this valve operates just like a normal check valve. Pilot line pressure holds the valve open for fluid in the reverse direction.
If the pilot line is pressurized (for example, if there is pressure at P0 in Fig. 7.10), the valve is held open for fluid flow from the press cylinder back into the reservoir. Some specific pilot pressure is needed to open the check. If the required pilot pressure is 33% of load pressure, the valve is designated a 3:1 pilot-operated check valve. When a pilot-operated check valve is used for the application shown in Fig. 7.10, it is typical to have it open with as small a pilot pressure as possible—say a 5:1 valve.