Pneumatic brake booster with reduced operational noise level

The servomotor has a shock absorbing unit (68) absorbing relative movement and oscillation between a control shaft (33) and a pneumatic piston (8). The shock absorbing unit comprises an annular joint (72) having a high friction coefficient and mounted in a circular groove (70). The groove is practiced on an exterior surface of a cylindrical part (58) of a plunger (32).