booster piston 6 will move to the left when viewing the drawing corresponding to the magnitude of the pressure metered into booster chamber 2 until a state of balance between pedal force and the restoring force of the master cylinder piston has been attained during such a movement stop element 23 remains in abutment with second booster piston 6 if the brake is applied and the auxiliary energy has failed booster chamber 2 remains unpressurized the pressure fluid enclosed in pressure chamber 19 prevents second booster piston 6 from travelling with yoke 14 to the left when viewing the drawing rather a movement of second booster piston 6 relative to piston rod 15 starts in the course of which stop element 23 lifts from second booster piston 6 and is immersed in pressure chamber 19 corresponding to the travel of yoke 14 in its turn the comparatively weak dimensioning of compression spring 18 has as a result that the additional pedal force which the driver has to produce is as low as possible besides the sealing material of piston bore 22 engaging at piston rod 15 hardly contributes to an increase of the pedal force required in the embodiment illustrated in fig4 second booster piston 6 and piston rod 15 are again rigidly coupled to.