Hydraulic booster

Claim 1

Hydraulic booster comprising: housing means defining a working chamber and a control chamber therein; a piston slidably mounted in said working chamber; valve means responsive to a fluid pressure level in said control chamber to shift from a released position venting said working chamber to communicate successively higher fluid pressure levels into said working chamber as said valve means is shifted from said released position; an output plunger operatively connected to said piston and slidably received within said control chamber; operator-actuated means including an input plunger slidably received within said control chamber filled with a hydraulic fluid; said input plunger having a greater transverse cross sectional area than the output plunger; said input plunger being so mounted in said control chamber to actuate said output plunger upon failure of fluid control level in said control chamber to permit direct mechanical actuation of said piston; said valve means having one of its faces exposed to the fluid pressure prevailing in the working chamber and the other of its faces exposed to the fluid pressure level prevailing in the control chamber; said valve means includes a spool and a control piston cooperating therewith, said control chamber including a portion receiving said control piston, said control piston having a reduced diameter with respect to the spool valve diameter; the control piston is telescopically mounted in the spool, said spool being sleeve-shaped to define an inner compartment, the control piston being adapted to be displaced in a limited manner with respect to the spool from a rest-abutting position into which said control piston is biased by a return spring, said control piston having a supply passage permitting the fluid under pressure to flow from the inner compartment of the sleeve-shaped spool to the working chammber, said supply passage being closed under the displacement of the control piston into the spool.