The invention relates to a pneumatic braking force amplifier for a motor vehicle, having an amplifier housing (1), the interior thereof being divided by at least one axially displaceable wall (2, 39) that can have a pneumatic differential pressure applied thereto into at least one working chamber (3, 41) and at least one vacuum chamber (4), having a control valve (12) disposed in a control housing and controlling the differential pressure for connecting the working chamber (3, 41) to the vacuum chamber (4) or to the atmosphere, wherein the displaceable wall (2, 39) comprises a diaphragm disc (8) and a rolling diaphragm (18, 38), and wherein the working chamber (3, 41) is sealed by means of a sealing ring (13, 47) contacting the control housing (5) and tensioned in the radial direction relative to a longitudinal axis L of the braking force amplifier. In order to prevent stick-slip and dynamic effects that arise in particular at the sealing ring and that cause background noise, according to the invention means are provided for reducing a surface of the displaceable wall (2, 39) that is effectively acted upon by subdividing the working chamber (3, 4) into a prechamber (30, 42) and a main chamber (31, 43) at the beginning of an actuation of the braking force amplifier.