Vacuum brake booster for an energy-assisted braking system for motor vehicles

The brake amplifier (10) has an input element (12) for control of a valve device (14) by an input force (F) for selective connection of a working chamber (18) to the atmosphere or a reduced pressure chamber (20), separated from the working chamber by a piston (24, for delivery of a brake pressure via an output element (26) coupled to the master cylinder. A reaction force dependent on the braking pressure is supplied to the input element by an elastomer reaction element (28) of the piston, a volume sensor (32) associated with the output element acting on the reaction element for altering its effective surface ratio.