a vacuum - operated brake power booster is provided having a movable wall which is sealed off relative to the vacuum housing and a control valve body which is slidable jointly with the movable wall . a hollow cylinder - shaped expanded plastic ring is arranged in the annular clearance between the neck portion of the pedal side housing shell and the collar of the diaphragm , which collar sealingly surrounds the control valve body . the expanded plastic ring effects the air flow during brake operation such that the air flow , enters the annular clearance through the duct extending in a radial direction within the control valve body and is suitably deflected and cushioned so that no noises arise at this point . in the same manner and simultaneously , vibrations , especially those of the neck portion of the housing shell of the vacuum housing , are suppressed , since the expanded plastic ring adheres to the inside wall of the housing shell .