Abstract: A pneumatic jack including a jack head adapted to be elevated by an expansible flexible bag member inflated with compressed air. A locking mechanism for holding the jack head in its elevated position, including a hydraulically controlled expansible plunger member connected to the jack head. The hydraulic fluid is introduced into the expansible plunger member through a check valve by the motive force of the compressed air. The jack head is lowered by relieving the compressed air within the bag member and by overriding the check valve to permit reverse flow of the hydraulic fluid from the extensible plunger member into a reservoir.