Brake boosting system

A brake boosting system comprising:

• a brake operating member for braking operation,
• an input shaft operated by the brake operating member,
• a reservoir for storing a brake fluid,
• a master cylinder having a master cylinder piston and actuated by the input shaft for producing a master cylinder pressure,
• a brake cylinder communicating with the master cylinder for producing a braking force when the master cylinder pressure is introduced into the brake cylinder,
• a pressure intensifying chamber facing one end of the master cylinder chamber,
• a reaction chamber disposed between the master cylinder piston and the input shaft and always communicating with the pressure intensifying chamber,
• a fluid pressure source for producing fluid pressure and supplying the fluid pressure into the pressure intensifying chamber, and
• communication control means which allows the pressure intensifying chamber and the reaction chamber to communicate with the reservoir when the system has not advanced and intercepts the pressure intensifying chamber and the reaction chamber from communicating with the reservoir when the input shaft advances, said communication control means having a solenoid control valve disposed between the pressure intensifying chamber and the fluid pressure source for controlling communication between the pressure intensifying chamber and the fluid pressure source.