Abstract: A percussion tool is hydraulically operated by a piston whose stroke is controlled by a spring biased valve. A pressure accumulator is connected to the working pressure chamber of a hydraulic cylinder and receives pressure therefrom during the return stroke of the piston while discharging pressure during the working stroke. A piston is displaceably mounted in the pressure accumulator for movement into an end position when the pressure is discharged and a second valve is actuated by the piston in the end position to control the spring biased valve.The invention refers to an improvement in hydraulic percussion tools.Hydraulic percussion tools, for rock drilling, breaking or the like of known construction have a reciprocating working piston in a cylinder, driven by a pressure liquid, usually hydraulic oil, during the reciprocating movement.