Abstract: An orifice and a one-way valve for permitting a flow of a liquid only in one direction from a primary master cylinder to an input port in a secondary master cylinder of a brake system are incorporated in parallel in an hydraulic pressure introduction passageway which connects the input port in the secondary master cylinder and an output port in the primary master cylinder. Thus, it is possible to moderate a kick-back phenomenon due to a back-flow of a pressure oil from a hydraulic pressure chamber in the secondary master cylinder via the input port toward the primary master cylinder, and to rapidly fill a brake oil in the secondary master cylinder from the side of the primary master cylinder.