A master cylinder is provided on the cylinder body with a reservoir-receiving mouth formed near the border of the section of of a large diameter and the section of a small diameter of a stepped bore, a through hole being formed between the bottom surface of the reservoir-receiving mouth and the first pressure chamber, a compensation hole being formed between the bottom surface of the reservoir-receiving mouth and the second pressure chamber, a valve seat fitted in and fixed to the reservoir-receiving mouth and having an internal hole formed therein, a check valve provided within the reservoir-receiving mouth between the bottom surface of the reservoir-receiving mouth and the valve seat with the valve element being pressed against a resilient member fixed to the valve seat by means of a spring, a projecting member protruding into the first pressure chamber through the through hole provided for the valve element coaxially with the valve element so that the valve element is titlted to form a passage between the valve element and the resilient member by engagement of the piston section of a small diameter of the stepped piston with the projecting member while the stepped piston is at a return position, and the valve element including a passage formed therein through which the first pressure chamber communicates with the reservoir via the internal hole formed in the valve seat and an relief valve for maintaining the passage closed until the effective pressure within the first pressure chamber reaches a predetermined value.