A brake booster wherein second and third pistons are separated within a bore of a master cylinder to define second and third chambers. Compensation valves control the flow fluid from a reservoir to the bore while a passage in the second piston connects the second chamber with the third chamber that is connected to the reservoir through an tilt valve that is held opened by pressurized fluid acting on a fourth piston. A control valve responds to a resiliently applied force from an input rod carried by the third piston to supply pressurized fluid to the brake system. In an event pressurized fluid is unavailable, a spring moves the fourth piston and a sixth spring to moves the tilt valve against a seat to seal the third chamber and create a hydraulic lock. Further movement of the third piston correspondingly moves the second piston to effect a manual brake application.