Hydraulic boosters for vehicle braking systems

Claim 1

An hydraulic booster for a vehicle braking system comprising a housing provided with a bore of stepped outline, an inlet for connection to a source of high pressure hydraulic fluid and an outlet for connection to a reservoir for fluid, a pedal-operated input member, a boost piston of stepped outline for actuating a master cylinder assembly working in said bore, and means defining a boost chamber in said bore, said boost piston being advanced in said bore in response to pressurisation of said boost chamber by fluid from said source, and valve means operable in response to a load applied to said input member to control pressurisation of said boost chamber, said valve means being operable to control communication between said inlet and said boost chamber and between said boost chamber and said outlet, wherein the diameters of said bore and said boost piston are arranged and sealed to provide an annular chamber surrounding said boost piston and connected to said inlet, such that a sealing diameter of said boost piston at a rearward end of said annular chamber is greater than a sealing diameter of said boost piston at a forward end of said annular chamber, with a radial clearance provided between said boost piston and said bore at said forward end of said annular chamber, and a forward seal between said bore and said boost piston at said forward end of said annular chamber is mounted in the wall of said bore such that said forward seal is radially deformable and is able to tolerate said radial clearance between said bore and said boost piston, said sealing diameters of said piston being chosen so that said boost piston can be assembled from the forward end of said bore, with said forward seal deforming radially to allow the passage of said greater rearward sealing diameter of said boost piston and returning to seal said lesser forward diameter.