A rotary air motor driven hydraulic pump has separate pressure and return actuators and first and second stage eccentric driven pistons 180 degrees out of phase and driven by the motor through a gear reduction unit. The pressure and return actuators are on/off/proportional valves and the pump has two pressure relief valves with one adjustable from outside of the pump for a user to set a pressure limit of the pump. Above a certain output pressure limit, a bypass valve shunts flow in excess of that needed to precharge the second stage compression chamber from the first stage piston to an elastic reservoir from which the hydraulic fluid pumped by the pump is drawn. To make the pump stall at a certain output pressure, a pressure limiting valve can be used that cuts off the air supply to the pump at the desired output pressure.