A pressure control reservoir includes a valve ball and a shaft which is movable to move the valve ball to open or close a fluid path leading to a fluid reservoir chamber. The shaft has a tip with a slant surface which has a peripheral edge of a polygonal shape with even numbers of vertices and is inclined to the longitudinal center line of the shaft. Such a polygonal geometry increases an entire or inclined length of the slant surface, which results in an increased range where an angle which the slant surface makes with a plane extending perpendicular to an axial direction of the tip is permitted to be increased without causing the point of contact between the valve ball and the slant surface to be shifted close to the tip of the slant surface when the tip of the shaft moves the valve ball.