The Bendix® DC-4000™ Double Check valve is used in an air brake system to direct a flow of air into a common line from either of two sources, whichever is at the higher pressure. The valves may be used for directing air flow for specific functions or to select the higher pressure of either of two sources of air as a supply source. Bendix manufactures multiple types of double check valves; one type is mounted in the air line without being secured to the vehicle frame, and another type contains a mounting tab to secure it to the vehicle frame or chassis. Although the valves are somewhat different physically, the same function is performed by each type.