Patent ID: 7077051

Claim:
A pneumatic brake booster comprising: a rigid casing; a moving partition for delimiting in a sealed way a front chamber and a rear chamber inside of said casing, said front chamber in operation being permanently connected to a first source of pressure having a first pressure; a pneumatic piston moving said moving partition and having hub mounted to slide in an opening in said casing; an operating rod that moves in said hub between a position of rest and an extreme actuating position as a function of a resultant of forces, said resultant of forces comprising an input force exerted in an axial actuating direction directed toward said front chamber and a main return force exerted by a main spring in an axial direction toward said rear chamber; a plunger located in a bore of said hub and carried along by said operating rod; and a three-way valve having a stationary annular valve seat formed on an internal periphery of said hub, a moving annular seat which is concentric to said stationary seat and borne by said plunger, and a tubular shutter concentric with said plunger and with said hub, said tubular shutter having an annular front shut-off face which co-operates selectively with said moving seat to isolate said rear chamber from a second source of pressure, said second source of pressure having a second pressure higher than said first pressure, said front shut-off face co-operating selectively with said stationary seat to allow said rear chamber to be connected to said second source of pressure, said plunger being characterised by a cylindrical body secured to said operating rod and a ring mounted to slide axially on said cylindrical body between a front stop and a rear stop, said ring having a front section located between said cylindrical body and said bore of said hub, said ring having a rear section which forms a collar on which said moving seat is defined, said front stop being separated from said rear stop by a distance which provides an axial excursion for said ring, and in that a secondary spring which exerts a secondary return force on said collar in said axial return direction to urge said collar toward said rear stop, and in that said ring and hub comprise first and second reversible retaining means for keeping said ring stationary with respect to said hub in opposition to said secondary return force acting on said ring to allow said rear chamber to be connected to said second source of pressure, said first and second retaining means being activated by said ring being moved into said bore over a travel distance which is equal to a minimum travel distance and being deactivated when said ring engages and moves said front stop in said axial return direction under the effect of said main return force.