Abstract: Servo unit provided with an internal bracing member which is stressed upon assembly, permitting mounting thereof to a vehicle by means of only two fixing screws. The bracing member has an initial shape which is curved in a given direction, forming a clearance between the inside face of the mounting wall and two regions of the bracing member against which the screws bear. The clearance is eliminated when the bracing member is pressed against the inside face at the moment of definitive fixing of the screws with respect to the casing. The bracing member of the invention provides for better distribution over the whole of the inside face of the braking reaction forces which are applied to the casing.