A brake system for a vehicle comprises a booster unit including a power piston disposed slidably in a housing for defining a first chamber and a second chamber in the housing, a regulator disposed slidably in the power piston, a flow restrictor formed between the first chamber and a fluid pressure source for restricting flow of fluid in response to movement of the regulator, a pilot pressure chamber disposed upstream of the flow restrictor, a movable member moved by the internal pressure of the pilot pressure chamber thereby permitting communication between the second chamber and the fluid pressure source, and a reaction spring disposed between the power piston and the regulator. A master cylinder is connected to the power piston of the booster unit to supply braking hydraulic fluid at controlled pressure to front and rear brakes, and a brake pedal actuates the regulator of the booster unit to cause movement of the regulator relative to the power piston.