A master cylinder reservoir includes a cylindrical body having an open upper end. A cap is fitted on and removably secured to the body, the cap engaging the open end of the body at a lower end portion thereof. The cap has a mounting portion formed adjacent to the lower end portion thereof. The mounting portion defines together with the upper surface of the open end of the body a circumferential space. The mounting portion has a first surface disposed in opposed spaced relation to the upper surface of the open end of the body. The mounting portion has a second surface disposed in opposed relation to the outer surface of the upper end of the body. The outer surface of the upper end is spaced from the second surface adjacent to the upper surface of the body to form a circumferential recess. A diaphragm received in the cap and having a peripheral portion received in the circumferential space and held between the first surface of the mounting portion and the upper surface of the open end of the body. The peripheral portion having a circumferential bead formed on a lower surface thereof, the bead being held in sealing engagement with the upper surface of the open end of the body. The peripheral portion of the diaphragm has an engaging section depending from its outer edge. The engaging section is received in the circumferential recess for engagement with the outer surface of the upper end of the body.