Hydraulic power booster

Claim 1

A hydraulic power booster for actuating a master cylinder in an automotive brake system comprising: a housing defining a bore and a booster chamber; a first master cylinder piston in said bore defining a first working chamber; a second master cylinder piston in said bore defining a second working chamber; a booster piston in said bore having an end at said booster chamber, said booster piston connected to said second master cylinder piston; brake valve means associated with said booster chamber for metering a pressurized fluid into said booster chamber as a function of an actuating force applied to a pedal-operated piston rod; an annular piston defining a first effective surface area mounted in said housing for slidable movement coaxial to said booster piston; stop means associated with said annular piston bearing against said housing in a released position of said piston rod; means associated with said booster piston and said annular piston providing for a limited independent axial movement of said booster piston relative to said annular piston over a predetermined amount of travel of said booster piston from said released position to an intermediate position of said piston rod, said predetermined amount of travel of said booster piston being an amount for displacing said first and said second master cylinder pistons a distance providing for delivery of a volume of brake fluid from said first and said second working chambers equal to a known absorption volume of said brake system; said means providing for a limited independent movement adapted to interconnect said booster piston and said annular piston for simultaneous movement in excess of said predetermined amount of travel, said stop means being displaced away from said housing during said simultaneous movement and said first effective surface area being exposed to said pressurized fluid; a central piston in said booster chamber defining a second effective surface area less than said first effective surface area, said central piston connected to said piston rod and mounted for slidable movement in said annular piston; and linkage means operatively connecting said central piston to said brake valve means.