Brake booster

During a normal brake operation, a brake reaction from a reaction disc (12) is transmitted to a valve plunger (15) through an outer plunger (17) and an inner plunger (18); during an emergency brake operation, as a valve plunger (15) is driven forward through a given stroke relative to a valve body (3), a tubular member (24) retracts relative to the valve body (3); in a servo balance condition which is reached subsequently, the valve plunger (15) retracts by an amount corresponding to the retraction of the tubular member (24), and the outer plunger (17) abuts against a holder (13); a brake reaction from the reaction disc (12) is transmitted to the valve plunger (15) only through the inner plunger (18), allowing a booster ratio to be greater and a jumping quantity to be greater than during. a normal brake operation, whereby it is possible to increase an output from a brake booster (1) rapidly in immediate response to a quick depression of a brake pedal.