Every reverse osmosis system equipped with an auto shut off valve requires a check valve to prevent backflow. This small but essential part plays an important role to the function of the RO system. It allows the system to shut off properly when the tan is full. Without a properly working check valve, a great deal of water will be wasted, and eventually the membrane will be ruined. This APEC quick connect check valve elbow is made of high quality plastic and is compatible with all APEC residential RO systems.