In a vacuum-operated brake power booster, a transverse member is provided which confines the axial movability of the control piston relative to the control housing. The transverse member abuts against a bead of a rubber-elastic material, the bead forming part of the slide-guide ring sealing the booster housing relative to the control housing. The tranverse member is formed of a substantially U-shaped sheet-metal blank, with both free ends of the fork arms extending parallel to two limbs of the member. The arms are provided with hook-shaped tabs which are bendable around a pin forming part of the control housing for the purpose of fixing the transverse member in its aperture in the control housing. In order to enable a proper and easy bending of the tabs during assembly, notches are provided at the radially outside small faces or edges of the two fork arms.