PURPOSE: To prevent a sealant from being encroached by a valve seat even in such a case as doing a pumping action by installing a valve, connecting the side of a pressure chamber of a valve element to the side of a reservoir according to the deformation of the sealant to the valve seat side, in space between a valve body and the sealant.

CONSTITUTION: A cylinder body 1 of a clutch master cylinder 51 has a cylinder hole 2, and a reservoir connection port 4 is formed at the closing end side, while a plastic piston 5 is inserted into the cylinder hole 2 free of slide motion, whereby a pressure chamber 6 is partitioned. A valve stem 17 is fitted in a front end of the piston 5 shiftably with an axial hole 5a of the piston 5, and a tubular valve body 18 is integrally installed in the tip of the valve stem 17. This valve body 18 is energized to the left by dint of spring force of a valve spring 21 in a gap with a cover member 24, and a through hole 18a is installed in a part being opposed to an interconnecting hole 12 of the cylinder body 1, thus it is freely connectible to the side of the pressure chamber 6 via a second through hole 18c.