Booster with 2 different thresholds and piezoelectrical actuator

The actuator has a piston (22) with a tubular bush (64) mounted around a plunger (56) forming front side of the piston. The piston has a piezoelectric actuator for increasing an initial length (L1) of the bush to a greater length. The length (L1) is increased to vary the position of the front side to push back a reaction disk such that a braking force is applied rapidly along a skip distance greater than a determined skip distance (d1). An independent claim is also included for a motor vehicle having a pneumatic actuator.