A brake booster for a wheeled vehicle having an engine including an exhaust braking system, a source of negative pressure, and a source of positive pressure, said brake booster acting in a hydraulic circuit connecting a master cylinder to wheel brake cylinders, comprising:
- a housing provided at one side thereof with a cylinder body including a first bore for interposition within said hydraulic circuit and a second bore in communication with said first bore;
- an operating piston being reciprocally disposed within said first bore for subdividing the interior of said first bore into first and second fluid chambers in fluid-flow communication with said master cylinder and said wheel brake cylinders, respectively, said operating piston including a check valve permitting fluid in said first fluid chamber to flow to said second fluid chamber when said operating piston is in a retracted position and preventing flow of fluid in said second fluid chamber to said first fluid chamber when said operating piston is translated from said retracted position;
- a biased diaphragm piston reciprocally disposed within said housing and subdividing the interior of said housing into a negative pressure chamber and a variable pressure chamber, said negative pressure chamber being in fluid-flow communication with said source of negative pressure and said variable pressure chamber being selectively connected for fluid-flow communication with said negative pressure chamber, with said source of positive pressure and with atmospheric pressure, said diaphragm piston having a push rod extending into said first bore for translating said operating piston from said retracted position to increase hydraulic pressure of fluid trapped within said second fluid chamber on closing of said check valve;
- a control piston movably disposed within said second bore of said cylinder body and being movably responsive to fluid pressure supplied by said master cylinder in to said second bore through said first fluid chamber of said first bore;
- first control valve means cooperating with said control piston for connecting in fluid flow communication said variable pressure chamber to said negative pressure chamber under inoperative condition of said control piston and for connecting in fluid flow communication said variable pressure chamber with atmospheric pressure in response to movement of said control piston, said first control valve means comprising:
- a biased diaphragm operatively connected to said control piston;
- a first valve seat provided on said diaphragm for controlling fluid flow communication between said negative pressure chamber and said variable pressure chamber;
- a second valve seat for controlling fluid flow communication between said variable pressure chamber and atmospheric pressure;
- and
- a first biased valve body including a pair of valve parts engagable with said first and second valve seats in response to movement of said diaphragm to selectively connect in fluid flow communication said variable pressure chamber, with said negative pressure chamber, and with atmospheric pressure;
- and
- second control valve means cooperating with said first control valve means for selectively connecting in fluid flow communication said variable pressure chamber with said source of positive pressure in response to displacement of said first control valve means, said second control valve means comprising:
- a third valve seat for controlling fluid flow communication between said variable pressure chamber and said source of positive pressure;
- and
- a second biased valve body including a valve part engagable with said third valve seat to sequentially connect in fluid-flow communication said variable pressure chamber with said source of positive pressure in response to operation of said first valve body.