A check valve, also referred to as a one way valve, is a type of valve that only allows air to flow in one direction. Check valves are found at the end of compressor leader lines, and are useful for isolating your air tank from your bags. Many will place a check valve between their air tank and manifold, which allows for the removal of the air tank while keeping air in the air springs. This also provides peace of mind knowing that a tank leak will not cause the air springs to slowly deflate over time.