Abstract: An air vacuum pump is provided with a plurality of venting passageways in order to balance internal pressures. The present invention is found in the context of a vacuum pump or decompressor of the type having an eccentric rotor with radial lamellae mounted thereon. As the rotor turns the lamellae form intermediate chambers which produce suction and compression. According to a preferred embodiment of the invention one passageway is located to vent the chamber having the biggest volume just downstream of the suction portion of the pump. More specifically the passageway is located such that the arc along the inner surface of the chamber of the stator between the passageway and the discharge outlet is equal to or greater than the arc between the two adjoining lamellae of the rotor which occupy that portion of the chamber.