toward a position in which the valve 53 blocks communication between the chambers 48 and 49 a bore 57 having a relatively large diameter is formed over both the blocks 22 and 23 an actuator or expansion piston 58 is slidably received in the bore 57 and is formed therein with a recess 60 an operating piston 62 is slidably received in the bore 57 concentrically with the expansion piston 58 and has a piston rod 64 fixed to the piston 62 and slidably received in the recess 60 the expansion piston 58 has an expansion chamber 66 at its right end in the drawing while the operating piston 62 has at its left end in the drawing a chamber 68 which communicates with the chamber 36 by way of a passageway 69 a chamber 70 is defined between the expansion piston 58 and the operating piston 62 and communicates with the atmosphere the expansion chamber 66 communicates by way of a check valve 72 with a chamber 74 which communicates with the chamber 46 by way of a passageway 76 the expansion chamber 66 also communicates with the chamber 48 by way of a passageway 78 the check valve 72 has a valve stem 80 fixed thereto and contactable with the expansion piston 58 for operating the check valve 72 the check valve.