Power-operated boosters

A pedal-operated booster for a vehicle braking system, comprising a housing, means defining a bore in said housing, an inlet port for connection to a fluid pressure source, an exhaust port for connection to a reservoir for fluid, an input piston to which an input force is applied, an output member through which an output force acts, a boost piston for augmenting said output force working in said bore, means defining a boost chamber in said housing behind said boost piston, said boost piston being advanced in said bore in response to fluid pressure from said source pressurising said boost chamber, and control valve means operative to control a supply of fluid from said source to said booster, said control valve means controlling pressurisation of said boost chamber in response to relative movement of said input piston and said boost piston, operation of said booster being in two stages, in a first stage, following small initial movement of said input piston in a brake-applying direction, said control valve means is operative to control said supply of fluid from said source to said booster to cause said output member to move through a limited distance independently of said input piston, and in a second stage, following further movement of said input piston in a brake-applying direction said control valve means is operative to control pressurisation of said boost chamber to cause continued movement of said output member in proportion to said further movement of said input piston, and said control valve means comprises relatively movable parts, said relatively movable parts including a control bore within said boost piston and a valve mechanism housed in said control bore, said valve mechanism having at least two valve parts, one of said valve parts being movable relative to said input piston, and having a pressure-responsive face, operation of said control valve means being dependent on pressure in said braking system, whereby in said first stage, said control valve means includes means whereby said braking system is pressurised to a threshold level by movement of said output member through an appropriate distance, and fluid pressure from said source is applied to said pressure-responsive face to move said one valve part relative to said input piston to place said boost chamber in communication with said inlet port.