24 d are connected to the fluid channels 24 e to 24 h by a respective bypass channel 26 furthermore a pressure controlled proportional valve 25 a 25 b 25 c and 25 d is inserted as a built in valve into each of the second bottom fluid channels 24 e to 24 h the valve closing the respective bypass channel 26 when the second pressure chamber 23 b is acted upon by hydraulic fluid having a predefined pressure from the hydraulic system 18 during the rapid approach the main piston 20 carries out a corresponding stroke movement between lower dead point ut and upper dead point ot and when the proportional valve 25 a to 25 d is open displaces the hydraulic fluid present in the first top pressure chamber 23 a via the first top fluid channels 24 a to 24 d the bypass channels 26 and the second bottom fluid channels 24 e to 24 h into the second bottom pressure chamber 23 b the first top pressure chamber 23 a the first bottom fluid channels 24 a to 24 d the bypass channels 26 the bottom fluid channels 24 e to 24 h and the second bottom pressure chamber 24 b thus form a closed hydraulic system which can be opened or closed depending on the position of the proportional valves 25 a to 25 d so that the first top pressure chamber 23 a and the second bottom pressure chamber 23 b are substantially depressurized and the hydraulic fluid is displaced into the second bottom pressure chamber 23 b of the main piston 20 and a fluid column.