Brake system with master cylinder and electric brake booster

The system (100) has an actuator piston (140) including a rear shoulder (151) forming a stop against which a thrust flange (143) of the piston rests, for driving the piston during normal operation. An assistance piston (150) freely advances with respect to the actuator piston under the thrust of a control rod (170) for actuating a piston (111) of a master cylinder (110) in case of failure of an electric brake booster (120). A return spring (163) is arranged between the thrust flange and the actuator piston.