Abstract: A hydraulic drive system for positioning a part such as the electrode of an electrical discharge machine includes a hydraulic cylinder and a piston slidably mounted in the cylinder and separating it into opposite first and second chambers on opposite sides of the piston. A piston rod extends from the piston through one end of the cylinder for connection to a part to be positioned. A reservoir containing hydraulic fluid is mounted on the cylinder, and a pump has an inlet communicating with the reservoir and an outlet connected via passageways to the first and second chambers. Return passageways are provided for returning fluid from the chambers to the reservoir, and valves control fluid flow from the pump to the chambers and back from the chambers to the reservoir in order to drive the piston in opposite directions. The reservoir, pump, valves, and at least part of each connecting passageway are located within a single outer housing.