[Summary] To provide a brake system in which due to change in structure of a seat member, an end portion of a push rod that faces a piston can be increased in diameter and thus weight reduction of entire construction of the piston and suppression of vibration hitting noises are obtained.A seat member 27 of a check valve mechanism 25 is formed at a position opposite to one open end of a connecting hole 27a with a recessed portion 27d that is opened to a liquid pressure chamber 20b, a retainer 31 that has at its center part a sliding hole 31c for slidably supporting the push rod 32 is fixed to an open side of the recessed portion, an annular part of the retainer is formed with an arcuate and elongate passage hole 31e for communicating the recessed portion with the liquid pressure chamber, a smaller diameter rod part 32a of the push rod, which is arranged at the side of the ball valve body 29, is inserted from the sliding hole toward a ball valve body through the recessed portion and the connecting hole, and a larger diameter rod part 32b of the push rod, which is kept inserted toward the ball valve body is slidably supported by the sliding hole.