Check valves are of several styles: swinging disc, ball, plunger, and poppet. Check valves of the pilot-operated type are also popular. The connection at the bottom is connected to a remotely operated pilot valve, As in the ordinary check valve, free flow is permitted in one direction when the line pressure lifts the plunger thus opening the valve. The flow is checked in the opposite direction until pilot pressure is applied to the blind end of the plunger through the pilot connection. This causes the plunger to unseat the valve spool and oil is allowed to flow through the valve.
When system pressure at the check valve inlet port is high enough to overcome the biasing spring force on the poppet, the poppet is pushed off its seat allowing oil to flow through the valve. This is known as the check valve's free flow direction. When the fluid attempts reverse direction of flow, the spring pushes the poppet back on the seat to block flow through the valve. Check valves can be direct acting or pilot operated. A pilot operated version is used where the no-flow characteristic of the valve is desired only for a portion of the system cycle. A pilot operated check valve also allows free flow from its inlet port to its outlet port just as in an in-line check valve.
The design and construction of a check valve makes it one of the simplest fluid power components. Though simple, its use can make the system sophisticated. By application of check valves one can also design many sophisticated circuit diagrams where a constant flow can be maintained in both directions by using them in a bridge.