Pneumatic brake booster, in particular for automobiles

The servomotor comprises an axial piston (20) interposed between a push rod (24) and a plunger (32) at the end of a control rod (22). A sensor (42) is mounted to slide on the plunger (32). There is tipping key immobilizer (62) axially locking the sensor relative to the plunger during normal braking and unlocking it during emergency braking so as to decrease the length of the plunger-sensor unit and increase the slewing phase.