Abstract: A vacuum power servo booster having an improved retainer for an annular seat of a control valve body. A spring seat for a compression spring which presses the annular seat against the valve housing is formed at the front end portion of a retainer. A recess is formed on the front side of the spring seat to receive the skirt of the control valve body. The spring seat is formed by bending a smaller diameter portion of the retainer inwardly prior to assembly. In this manner, the longitudinal axis of the retainer is made to substantially coincide with that of the control valve body.