Abstract: An improvement of a recess formed in one end of an output shaft of a brake booster is disclosed. At its one end, the rear extremity of the output shaft is formed with a flange-like stop, which is formed of an annular sheet material. The engagement of the stop with the retainer is effective to prevent said one end of the output shaft from being disengaged from an annular projection on a valve body. This allows the diameter of a reaction disc to be increased without requiring an increase in the diameter of the valve body. Accordingly, a brake feeling can be adjusted over an increased extent than before by increasing the diameter of the reaction disc.