A low-pressure casing for a brake booster for automotive vehicles comprising two casing shells. A central ring flange is located at the inner surface of the end wall of each of the two casing shells. The portion of each end wall lying outside the ring flange is of truncated-cone-shaped construction and merges into a cylindrical circumferential casing section. At each of these merger points, a prop ring is inserted to bear against the inner surface of the associated one of the two casing shells.