Abstract: A cartridge concentrically received within a bushing fixed to the end wall of an air reservoir tank. The bushing includes a retaining groove for receiving a snap ring engageable with a retaining surface on the cartridge, thereby securing the cartridge within the end wall of the tank. The cartridge includes a fluid passageway which may be provided with a plurality of components, including push-to-connect tube or hose fittings, inlet check valves, pressure relief valves, or the like, by which the cartridge provides for communication with the interior of the air reservoir tank.