is mounted on the lower end portion of the outer periphery of the ring member 14 and the lower end portion of the anchoring member 16 is loosely inserted into a through hole 9a in the support 9 a stop 17 fixed to the support 9 is disposed between leg portions 16a of the anchoring member 16 and is adapted to stop the rotation of the ring member 14 the stop 17 is substantially circular in cross section and has a flat bottom surface where it is secured to the support 9 an actuating lever 18 is securely mounted on the shaft 11 between the actuating lever 18 and the rocking arm 7 is disposed a connecting rod 19 which is disposed substantially in parallel with the piston cylinder assembly 8 at the upper end portion of the connecting rod 19 is provided an elongate hole or a control portion 19a which extends in the longitudinal direction of the connecting rod 19 a pin 20 is mounted on that intermediate portion 7c of the rocking arm 7 which is situated between the shaft 6 and the rear end portion 7b of the rocking arm 7 the pin 20 is loosely fitted in the elongate hole 19a referring back to fig1 and 2 a.