Automotive brake booster device

An automotive brake booster device of the type including a booster shell, a booster piston slidably accommodated in the booster shell, a first working chamber defined in the booster shell by the booster piston on the front side thereof and continuously held in communication with a source of vacuum supply, a biasing means for urging the booster piston in a retracting direction, a second working chamber defined in the booster shell by the booster piston on the rear side thereof, an input rod connected with a brake pedal and arranged opposite the booster piston for movement toward and away from the latter, control valve means for selectively placing the second working chamber in communication with the first working chamber and the external atmosphere, said control valve means being arranged between said input rod and said booster piston and being operable upon forward movement of the input rod to produce a pressure differential between the first and second working chambers which is effective to cause the booster piston to follow the movement of the input rod, and an output rod operatively connected with the booster piston and held in a position opposite the operating piston of a brake master cylinder, said device comprising adjustable stop means for limiting the extent of retracting movement of the input rod in such a manner that, with the input rod fully retracted, said control valve means is held in a neutral position to keep the second working chamber out of communication with the first working chamber as well as with the external atmosphere while the output rod is held in direct abutting engagement with the operating piston of the brake master cylinder.