A vacuum brake servo for motor vehicles has a servo housing (1) with a movable wall (2) and a control housing (9) that carries the movable wall (2). The control housing (9) has a main part (13) arranged in a guide pipe (15) together with a cover part (14) axially inserted on the main part (13). An electromagnet (11) for driving a control valve (8) is arranged in the control housing (9). One axial side (16) of the electromagnet (11) is mounted in the main part (13) and its other axial side (17) is mounted in the cover part (14). The cover part (14) is provided with axially projecting centring segments that elastically deform and centre in a play-free manner the main part (13) when the control housing (9) is assembled.