Check valves will prevent flow in one direction and facilitate flow in the opposite direction. Check valves are sometimes called non-return valves. Normally Closed style check valves have a spring and disk combination that will open once the pressure differential is greater than the spring force. The pressure differential that a normally closed check valve opens is called the "Cracking Pressure". Air Logic's normally closed check valves combine the benefits of a soft silicone seat with a hard floating disk: great sealing characteristics and high reverse pressure ratings. Reference the Air Logic 10-32 thread interface and seal guide.