Hydraulic brake booster

In a hydraulic brake booster, a housing defining a pressure chamber, a piston movably disposed within the housing in response to fluid pressure communicated to the pressure chamber from a fluid pressure source, a control valve assembly movably disposed within the housing to control communication of said fluid pressure to the pressure chamber, the control valve assembly movably carrying a sleeve within the pressure chamber, an input member engageable with the sleeve to move the sleeve and control valve assembly and valve means carried by the control valve assembly within the pressure chamber to open communication between the pressure chamber and said fluid pressure source, characterized by said valve means defining a first position in engagement with said sleeve to provide a first opening leading to the pressure chamber, said valve means defining a second position spaced from said sleeve to provide a second opening leading to the pressure chamber, said second opening being different in size than said first opening and said valve means defining a third position substantially closing communication to the pressure chamber.