the desired length 34 1 34 and a bushing 52 the bushing 52 guides the rod 53 and engages the front part 24 at annular surface 47 one component of the preselected input force is applied to the rod 53 upon the evacuation of the booster housing at the same time the other component of the force is exerted on the bushing 52 and transmitted to the front part 24 through contact with the annular surface 47 sliding the front part 24 in the sense of venting of the power chamber 23 the pre determined force component exerted on the rod 53 is transmitted directly to the transmission ratio disc 6 as mentioned above the measurement of the output force also takes place simultaneously the position of the rod relative the front part 24 is adjusted until the output force is within the pre determined range the relative movement of the rod 53 over the bushing 52 from a starting point at the annular surface 47 in achieving the pre determined output force is measured as by a dial gauge 54 this thus provides a measure of the distance between the annular surface 47 and the front part 24 and the abutment.