Abstract: A hydraulic power pack for supplying hydraulic fluid under pressure for operating apparatus such as hydraulic workover units used in servicing oil and gas wells. The power pack provides substantially constant hydraulic power input to the driven apparatus over a wide range of pressures and flow rates. The power pack includes a constant speed engine, a gear box driven by the engine, a bank of positive fixed displacement pumps of varying sizes driven by the gear box and having discharge ports connected with a common discharge manifold, a hydraulic fluid reservoir, an intake manifold between the reservoir and the pumps, a return line to the reservoir, a recirculating line connected between the discharge from each of the pumps and the return line, and an unloading valve in each of the recirculating lines operable responsive to the pressure in the discharge manifold for recirculating each of the pumps to the reservoir at a predetermined pressure value.