Hydraulic brake booster with shut-off means

A motor vehicle fluid power circuit comprising a pump, a steering valve, and a brake booster, said pump having an inlet and an outlet and a relief valve hydraulically connecting said inlet to said outlet at a predetermined relief pressure, said brake booster having a power chamber, means hydraulically connecting said pump outlet and inlet to said steering valve and further means hydraulically connecting said pump outlet and inlet to said power chamber, said further means including an inlet passage in said brake booster hydraulically connecting said pump outlet to said power chamber and an outlet passage in said brake booster hydraulically connecting said pump inlet to said power chamber, a first valve surface in said brake booster movable in said outlet passage relative to said power piston between an open position and a closed position for opening and closing said outlet passage, a second valve surface in said brake booster movable in said inlet passage relative to said power piston between an open position and a closed position for opening and closing said inlet passage, shut-off means for moving said second valve surface relative to said power piston from said open position to said closed position to close said inlet passage and completely block said inlet passage to hydraulically isolate said pump outlet from said power chamber in response to the fluid pressure in said power chamber reaching a predetermined shut-off pressure, said shut-off means including a member movable relative to said first valve surface and having a fluid pressure surface exposed to fluid pressure in said power chamber and means communicating said fluid pressure in said power chamber with said fluid pressure surface, said member being operatively connected to said second valve surface to move said second valve surface relative to said power piston to said closed position when said fluid pressure in said power chamber reaches said shut-off pressure, said first valve surface being in said closed position to close said outlet passage and completely block said outlet passage to hydraulically isolate said fluid power chamber from said pump inlet when said member fluid pressure surface operates to close said second valve surface in response to said shut-off pressure, whereby said first and second valve surfaces prevent fluid flow through said power chamber from said inlet passage to said outlet passage when said shut-off pressure is reached, and said shut-off pressure being less than said relief pressure of said pump relief valve.