Vacuum brake booster, especially for a vehicle

The servomotor has a push rod (32) with a thermal dilatation unit (48) mounted between a stopper (50) integrated to the rod, and a front radial side (42) on a reaction disk (30). An axial gap is provided which determines a point of changing of power assistance ratio. The dilatation unit (48) provides an effect opposite to that of the disk on the axial gap at the time of temperature variations.