The booster has a housing (2) defining a pressure chamber (4,5), and a movable wall (3) in the housing to divide the pressure chamber into one chamber of constant pressure, communicating with a vacuum source and one chamber of variable pressure which selectively communicates with the atmosphere or with the vacuum source. A movable piston (6) is connected to the movable wall, and an input element (7,8), in the piston, can move in its axial direction on the basis of brake pedal activity. An output element (10) provides a forward force from the piston responding to a movement of the movable wall and which in its axial direction is subdivided into a first element (101) that lies on its input side and a second element (102) that lies on its output side. A control element (13) brings the variable pressure chamber into communication with the vacuum source or the atmosphere corresponding to a movement of the input element against the movable piston. A movable auxiliary wall (17), in the constant pressure chamber, engages the second output element during a movement of the movable auxiliary wall. An isolating element (18), in engagement with the auxiliary wall, forms an auxiliary variable pressure chamber, works with the auxiliary wall. A connecting chamber switch element (21,22) allows the auxiliary variable pressure chamber communicates with the vacuum source or the atmosphere.
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