Abstract: A vacuum brake power booster is described wherein the return movement of a valve piston actuating a control valve is limited by a U-shaped element (17) slidable into a control housing (11). In order to improve the ventilation of the working chamber of the brake power booster initiated on actuation and to shorten the booster's response time, exit areas (42, 43, 44, 45) of ventilation ducts (38, 39, 40, 41) permitting an air flow into the working chamber (3) are formed radially outwardly of an area in the control housing (11) between the arms of the U-shaped element (17), the exit areas being confined by the outside walls (34, 35, 36, 37, 46, 47, 48, 49) of air evacuation ducts (28, 29, 30, 31) interconnecting the working chamber (3) and a vacuum chamber(4).