Abstract: A brake booster for holding the braking action even if the brake pedal is released. The booster has a low pressure chamber and a variable pressure chamber with a diaphragm therebetween for moving the output shaft to the brake cylinder. A valve mechanism mechanically controlled by the brake pedal switches a communication passage between the low and variable pressure chambers and an atmospheric pressure passage from the variable pressure chamber to the exterior. A first solenoid valve can close the communication passage between the low and variable pressure chambers irrespective of the brake pedal position so as to hold the braking. A second solenoid valve can control a passage between the variable pressure chamber and the low pressure source so that the braking release can be gently effected.