Brake booster

Claim 1

A brake booster comprising a housing (2,3) defining an interior space, diaphragm means (4) connected to said housing in said interior space and dividing said interior space into a constant pressure chamber (5) and a variable pressure chamber (6), a power piston (7) movable in said housing and connected to said diaphragm means for moving said diaphragm means in said space, a push rod (16) for pressing a master cylinder of a brake, reaction means (10) operatively engaged between said power piston and said push rod for transmitting pushing forces from said power piston to said push rod, a vacuum valve (13) in said power piston for opening and closing communication between said constant pressure chamber and said variable pressure chamber, an open air valve (14) in said power piston for opening and closing communication between said variable pressure chamber and the ambient atmosphere, a relay rod (11) movably mounted in said housing and operatively connected to said vacuum valve and said open air valve for opening and closing said vacuum and open air valves with movement of said relay rod, said relay rod having one end which is engageable against said push rod through said reaction means, an operating rod (18) adapted to be pushed by movement of a brake pedal of a brake, said operating rod being engaged with said relay rod for moving said relay rod, a hollow expansion body (22) disposed in said constant pressure chamber and connected between said housing and said push rod, said expansion body defining an expansion chamber (25) and being expandable and contractable in an axial direction of movement of said push rod in said housing, and pressure valving means connected to said housing and between said expansion chamber, said constant pressure chamber and said variable pressure chamber, said pressure valving means having a plurality of positions for establishing varied communication between said expansion chamber and at least one of said constant pressure chamber and said variable pressure chamber so that an inner pressure of said expansion chamber can be changed dependent on a pressure in one of said constant pressure chamber and said variable pressure pressure chamber to change a boosting ratio of said brake booster, said valving means positioned externally of said housing space.