Apparatus and method for pre-filling a hydraulic motor

A reservoir for holding pre-fill hydraulic fluid is connected at its bottom to the power input side of a hydraulic motor. A stand pipe within the reservoir is connected to a hydraulic fluid drain outlet for expulsion of excess volume hydraulic fluid. Compressed air is introduced to the reservoir at a level above the top of the stand pipe. Valving in the compressed air line is operable to three states: opened between the compressed air source and the reservoir; opened between the reservoir and the atmosphere and closed. A valve connected in the drain outlet may be opened or closed. A check valve between the reservoir and the hydraulic motor is held opened by pilot operation during return of the hydraulic fluid to the reservoir and checked closed during high pressure input to the hydraulic motor. Control means are provided for transmitting fluid under relatively low pressure to the hydraulic motor during its closure stroke and for causing said fluid to be returned to the reservoir during the retraction stroke with any excess volume hydraulic fluid being rapidly expelled through the drain outlet with a vortex being formed at the top of the stand pipe as the excess fluid enters therein.