A hub cap for a wheel hub comprises a cylindrical housing having a closed first end and an open second end, the open second being adapted for attachment in an air-tight sealing arrangement to the outer end of a wheel hub to form a closed air system in the interior of the wheel hub; an air valve in an air flow connection with the closed air system for receiving pressurized air; and an air pressure gauge in an air flow connection with the closed air system operable to provide a visual indication of the air pressure in the closed air system whereby a breached seal condition within the wheel hub can be detected. The second end is formed with a flange and an outer annular shoulder wherein the flange is adapted to fit into the outer end of a wheel hub.
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