PURPOSE: To minimize the ineffective operation of a pneumatic brakebooster, to make the jump adjustable to a desired value from outside after assembling the booster and to enable the brakebooster to operate quietly.

CONSTITUTION: A first valve seat 136 is formed on a plunger 32 connected with a control rod 34. A second valve seat 140 is formed on a skirt 14 supported by a rear tubular part 22 of a piston. A shutter 138 formed by a flexible tubular diaphragm 114 operates interactively with both the valve seats 136 and 140. The plunger 32 consists of a front part 126, a rear part 124 screwed to the front part and an active part 128 forming the first valve seat. The front part can be displaced in the translation motion direction against the active part 128 by rotating relatively the front and the rear parts.