between the cover plate 26 and the piston 20 there is provided a spring 38 for urging the piston 20 and piston rod 27 in the axially inward direction the piston rod 27 has an end plate 24 mounted on the outer end thereof by a nut 41 and the end plate 24 has an axially inner annular flange portion 24a which is received on the axially outer portion 19c of the bobbin holder 19 between the annular flange portion 24a of the end plate 24 and an flange portion 19e formed in the axially inner end of the cylindrical bobbin holder 19 there are interposed a first elastic body 21a and a first sleeve 22a which are received on the axially outer portion 19c of the bobbin holder 19 and a second elastic body 21b a second sleeve 22b a third elastic body 21c a third sleeve 22c a fourth elastic body 21d and a fourth sleeve 22d which are received on the axially inner portion 19b of the bobbin holder 19 an outer bobbin 36a and an inner bobbin 36b are received on the leg portions of the elastic bodies extending radially outward as stated above the end plate 24 is urged in the axially inward direction by.