A pneumatic brake servo for motor vehicles having a servo housing, the interior of which is divided into at least one working chamber and at least one vacuum chamber by at least one axially movable wall which can be loaded with a pneumatic differential pressure, having a control valve which controls the differential pressure and is arranged in a control housing for connecting the working chamber to the vacuum chamber or to atmosphere. The movable wall includes a diaphragm plate and a rolling diaphragm, and the working chamber is sealed by a sealing ring which bears against the control housing and which is clamped in the radial direction in relation to a longitudinal axis of the brake servo. An actively loaded area of the movable wall is reduced by division of the working chamber into a prechamber and a main chamber to avoid stick/slip effects and reduce disruptive noises.