A compressor includes a housing having an upward extending tube coupled to a pressure gage. A piston is slidably received in the housing and has an annular groove for loosely receiving a sealing ring and has one or more notches communicating with the annular groove of the piston. A gear is driven by a motor and coupled to the piston with an eccentric pin for moving the piston along the housing in a reciprocating action. The gear has one half of greater teeth than that of the other half for increasing the driving effect of the compressor.