Hydraulic system for a differential piston type cylinder

A hydraulic system for a hydraulic cylinder of the differential piston type, comprising pump means for delivering fluid from a reservoir to the cylinder chamber which volume increases when the piston moves, wherein the fluid displaced from the opposite cylinder chamber is pressurized to build up a counter-pressure in this cylinder chamber to provide a hydraulic bias to said piston, characterized in that the pump means comprises a first variable displacement pump and a second variable displacement pump, both cylinder chambers being connected to each other via said first variable displacement pump and that the cylinder chamber larger in volume is connected to said reservoir via said second variable displacement pump, and means for adjusting the delivery volume of said second variable displacement pump relative to said first variable displacement pump in response to a predetermined speed of said piston so that (1) the volume of said second variable displacement pump delivers to the larger volume chamber is larger than the volume determined by the differential area between the cylinder chamber volumes when the piston moves toward the lower volume chamber, and (2) delivers a smaller volume to the larger volume chamber when said second variable displacement pump returns fluid from the larger volume chamber to the reservoir when the piston moves in the opposite direction.