A simple valve that allows fluid to flow in one direction, but prevents it from flowing in the other. Such valves can be useful in all kinds of fluid systems, and are available in all sizes, for such applications as priming your kitchen sink faucet (tap), bleeding brakes and fuel lines, and removing spilled fluid through suction.

To use, just place the valve between the source and destination (for example, between water pump and faucet. Operation is automatic, with fluid movement.