on top thereof is connected to a piston 16 the cylinder 2 is located in an aperture 211 of base 21 which includes a reduced portion 21 2 outside the aperture 211 two oil seals 213 214 are inserted into the reduced portion 212 and the aperture 211 for preventing oil leakage the cylinder piston 3 is also sealed with oil seal 33 to prevent oil leakage from the cylinder 2 the bottom portion of the cylinder piston 3 has a piston rod 31 extending through the aperture 211 and into the base 21 piston rod 31 has a bore 311 adopted to communicate with the pressure adjustment valve 4 the pressure adjustment valve 4 according to fig2 b and 2c is composed of a block 41 on top having an aperture 411 at its center and male threads 412 which are adapted to adjust the tension of a spring 42 the spring 42 urges a ball 43 downward to seat on top of the bore 311 to either close or to open the bore 311 the control valve 5 includes a groove 51 which forms a clearance 511 between the groove 51 and the cylinder piston 3 when the cylinder piston 3 moves to its top position at the top center there is a female threaded hole 52 which has a passage 53 at one side which communicates with the hydraulic oil tank 6 valve 54 is threadedly inserted into the female threaded hole 52 and engages a ball 55 thereof the ball 55 engages the passage 53 which.