Brake system for vehicles

A brake system for a vehicle comprising:

• booster means including a cylinder, a piston disposed for sliding movement in said cylinder, a pressure chamber defined between said cylinder and said piston, and a valve member arranged for interlocking movement with a brake pedal and disposed slidably in said piston for controlling flow of pressure fluid from a fluid pressure source into said pressure chamber;
• pressure control valve means disposed between said fluid pressure source and said booster means for controlling the maximum internal pressure of said pressure chamber of said booster means depending on the weight of the vehicle;
• a master cylinder connected to said piston of said booster means to supply braking hydraulic fluid at controlled pressure to the front and rear brakes said booster means including a discharge chamber defined on the opposite side of said pressure chamber relative to said piston, said valve member permitting communication between said discharge chamber and said pressure chamber when the brake pedal is not actuated, but permitting communication between said pressure chamber and said fluid pressure source when the brake pedal is actuated, and wherein a second pressure control valve means communicates with said discharge chamber of said booster means, biasing means engaging said second pressure control valve means to urge it towards its closed position, and drive means is provided for controlling the valve closing force produced by said biasing means to urge said second pressure control valve means towards its closed position in a relation inversely proportional to the weight of the vehicle.