Abstract: A vehicle air supply system (200) providing pressurized air from an axle (12) of a vehicle to rotating tires to maintain a desired pneumatic pressure. The air supply system is adapted to be sealingly disposed within the hollow axle of the vehicle, having a rotary union (RU) disposed therein protecting the rotary union from external conditions such as oil leaks or brake shavings. The air supply system includes an axle plug assembly (300) which is seated within the hollow axle (12), and then having resilient members expanded therewithin to provide an air tight seal. The axle plug assembly includes a slidable piston member (260) is axially movable within a plug assembly bolt (282) to facilitate adapting the device to vehicles having different axle lengths. A graphite seal (250) is provided to minimize the friction between a rotating shaft coupled to the oil cap (202), and the stationary axle plug assembly.