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This work describes an efficient means to adjust the power level of an axial piston hydraulic pump/motor. Conventionally, the displacement of a piston pump is varied by changing the stroke length of each piston. Since the losses do not decrease proportionally to the displacement, the efficiency is low at low displacements. Here, with partial-stroke piston pressurization (PSPP), displacement is varied by changing the portion of the piston stroke over which the piston is subjected to high pressure. Since leakage and friction losses drop as the displacement is decreased, higher efficiency is achieved at low displacements with PSPP. While other systems have implemented PSPP with electric or cam-actuated valves, the pump described in this paper is unique in implementing PSPP by way of a simple, robust hydro-mechanical valve system. Experimental testing of a prototype PSPP pump/motor shows that the full load efficiency is maintained even at low displacements.