Power brake unit

A power brake unit for automotive vehicles comprising:

• a housing having therein first and second parallel bore bearing booster pistons respectively therein, a booster chamber common to said first and second bores and an intermediate pressure chamber bounded on one side by an intermediate piston adapted to be coupled to a master cylinder piston and on the other side by a first of said booster pistons;
• an axially displaceable booster piston arrangment separating said booster chamber from said intermediate pressure chamber, valve means for applying pressure to said chamber to move said piston arrangement, said booster piston arrangement including said first of said booster pistons disposed in one of said first and second bores, a second of said booster pistons disposed in the other of said first and second bores and means interconnecting said first and second booster pistons for movement of said pistons responsive to any actuating force applied to the booster pistons, said interconnecting means including a yoke affixed to said first booster piston for like movement therewith, a piston rod extending through said yoke for movement relative to said yoke, said piston rod affixed to said second booster piston, a compression spring bearing at one end against the second booster piston and at its other end against the inward side of said yoke to limit the movement of the second piston in response to movement of said first piston alone in a brake operating direction, and a stop on said piston rod externally of said yoke to provide a positive engagement between said first and second booster pistons in a brake release direction independent of pressure from a pressure source;
• and
• a hydraulic transmission ratio arrangement including said intermediate chamber effective between a brake pedal and the intermediate piston coupling to the master cylinder piston, said transmission ratio arrangement being disconnectable upon failure of said pressure source.