How does a check valve work?

There are basically 2 kinds . The first, used a lot in septic and irrigation lines is the flapper check valve . this is a tube containing a simple flapper on a small hinge that opens to flow direction and closes by gravity when flow drops. These are usually plastic valves and are relatively cheap.

The other type, in use in Plumbing systems is the spring loaded check valve, These are usually well made in brass casing and hold a round stopper that seals against a round surface. At back of the stopper is a small spring that keeps the valve closed till pressure against it gets to a set level, usually about 25psi. When pressure drops, the valve closes.