To reduce an amount of oil leaking to an oil discharge port through the outer peripheral surface of a piston by a method wherein a cylinder member fitted in an oiltight manner in the inner peripheral surface of a cylinder case is located between the inner peripheral surface of a cylinder case and the outer peripheral surface of a piston and an opening part connected to the oil discharge port is formed therein.

When a piston 63 effects stroke within a cylinder case 64, viscosity of pressure oil in an oil pressure chamber between the bottom of a recessed part 64b and the inner end face of a piston 63 is reduced due to the increase of temperature and the pressure oil is about to leak through the inner and outer peripheral surfaces of the piston 63 to the outside of the cylinder case 64 but the leak is stopped by O-rings (b) and (c). The pressure oil leaking to the outer peripheral side of the piston 63 flows through the opening part 40 of a cylinder member 39 before the flow of it to the O-ring (b). But, when a brake is in an engaging state, since the same pressure oil as that in the recessed part 64b is exerted on the opening part 40, no leakage occurs. Further, when a brake is in a state to disengage a clutch, the opening part 40 is communicated with the tank side through an annular groove 65a, since the degree of the opening of the opening part 40 is reduced by a cylinder member 39, leaking pressure oil is reduced to a minimum.