A vehicle brake booster system includes a reservoir, a pump, and a brake booster. The brake booster includes an input member adapted to receive a manual braking input force and an output member adapted to apply a braking output force to a master cylinder. A supply line couples the pump to the brake booster, and a return line couples an outlet of the brake booster to the reservoir. A flow of hydraulic fluid pumped to the brake booster provides a boost factor by which the braking output force exceeds the braking input force. The brake booster is hydraulically lockable by a first valve in the supply line and a second valve in the return line such that when the first and second valves are closed, a quantity of hydraulic fluid is trapped within the brake booster to maintain operation of the brake booster without continued operation of the pump.

An input piston connected to a brake pedal and an output piston which is moved forward to increase a hydraulic pressure in a pressure chamber are fitted to each other telescopically, and an engaging spring is provided between the input piston and the pressure chamber. The output piston is moved forward relative to the input piston by a hydraulic pressure in a rear chamber while compressing the engaging spring. Since a set load of the engaging spring is set at a small value, the output piston is moved forward relative to the input piston in an initial period of a braking operation, making it possible to reduce a stroke of the input piston which is required for a hydraulic pressure in brake cylinders to reach a first set pressure.

A brake unit (1) includes: a master cylinder (2) unit which is fixed to a vehicle body, and which includes a master cylinder (2b) that is received within a first housing (20), and that is arranged to be interlocked with a brake pedal; and a pump unit (3) which is arranged to be driven by a motor (3), and which includes a second housing (30) within which a pump arranged to increase a wheel cylinder pressure is received, the master cylinder unit (2) and the pump unit (3) being integrally fixed through a mount (6) which is an elastic member.

This hydraulic braking device is provided with: a tubular cylinder; an assisting piston, which is housed in the cylinder so as to be able to move reciprocally, faces a reservoir formed inside the cylinder, and assists a master piston, in a master cylinder, that generates hydraulic pressure for generating a braking force by reciprocally moving from one side to the other as a result of a fluid flowing into the reservoir from a supply source; and a seal member, which is elastically deformable and which seals the reservoir. When the assisting piston is at a location on one side, the reservoir is compartmentalized into a first chamber that communicates with the supply source, and a second chamber that communicates with the first chamber through a restricting flow passage, and the seal member seals the second chamber.

A vehicle brake booster system includes a reservoir, a pump, and a brake booster. The brake booster includes an input member adapted to receive a manual braking input force and an output member adapted to apply a braking output force to a master cylinder. A supply line couples the pump to the brake booster, and a return line couples an outlet of the brake booster to the reservoir. A flow of hydraulic fluid pumped to the brake booster provides a boost factor by which the braking output force exceeds the braking input force. The brake booster is hydraulically lockable by a first valve in the supply line and a second valve in the return line such that when the first and second valves are closed, a quantity of hydraulic fluid is trapped within the brake booster to maintain operation of the brake booster without continued operation of the pump.