Hydraulic boosters for vehicle braking systems

Claim 1

An hydraulic booster for a vehicle braking system, comprising a housing provided with a bore, an inlet for connection to a source of hydraulic pressure fluid and an outlet for connection to a reservoir for fluid, a pedal-operated input member, an output member, and a boost piston working in said bore, said boost piston being advanced in said bore in response to pressurisation of a boost chamber by pressurised fluid from said source, valve means for controlling pressurisation of said boost chamber, said valve means being operative to control communication between said inlet and said boost chamber, and between said boost chamber and said outlet, and said valve means being operable in reponse to relative movement of parts of said booster on movement of said input member, and said output member is movable relative to said boost piston through a distance determined by a limiting means, and an auxiliary pressure chamber is defined between said boost piston and said output member, the area of said boost piston on which pressure in said auxiliary chamber acts being greater than or equal to the area of said boost piston on which pressure in said boost chamber acts, and said booster has two stages of operation, in a first stage following an initial movement of said input member, said valve means is operative to supply pressurised fluid from said source to said auxiliary chamber, whereby said output member is advanced through said distance determined by said limiting means, without said boost piston being advanced in said bore, and in a second stage following further movement of said input member said limiting means is operative to ensure that said output member and said boost piston move together in response to pressurisation of said boost chamber.