A valve for an automotive brake master cylinder piston is disposed at the end of forward stroke of the brake master cylinder piston in a passage defined along the axis of the master cylinder piston and communicating mutually an oil reservoir and an oil outlet, and is adapted to close and open the passage when abutting upon the departing from a side wall surface of the end of forward stoke of the master cylinder piston in accordance with a forward stroke motion and a return stroke motion of the master cylinder piston. The valve comprises an extension rod extending into the opening of the passage, the extension rod being formed with a first restricting at the leading end thereof extending slidably into the opening of the passage, and also formed with a second restricting at the opposite root portion thereof adapted to operatively engage with the inner circumferential edge of one end of the passage through a tapered surface thereof, when the valve is closed, and to guide the valve to be shifted in the axial direction in such a manner that the first and second restricting may have the axis of the valve coincide with the axis of the passage at the time of closing of the valve.