To provide a sufficient booster function with a simple structure even after a negative pressure type booster device reaches a boosting limit (full load point) or even when a negative supply pressure lowers due to defect of negative pressure source of the negative pressure type booster device.
A brake system 1 comprises a negative pressure type booster device 3, a master cylinder 4, and booster means 12. The booster means 12 comprises a booster chamber provided on the rear side of a primary piston 33 of the master cylinder 4, a pump 71 feeding brake fluid to the booster chamber, and a reaction piston 35 and a sleeve controlling the fluid pressure of brake fluid supplied to the booster chamber. When a brake pedal 2 is depressed and the booster means 12 is operated, the primary piston 33 is operated by an energizing force provided by totaling an energizing force of the negative pressure type booster device 3 and an energizing force by the booster means 12 so as to produce a master cylinder pressure. This total energizing force is formed so as to be in proportion to the pressing force (input) of the brake pedal 2.
TAKASAKI YOSHIYASU
SHIMADA MASAHIRO
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