Spool valve for hydraulic brake booster

In a hydraulic brake booster:

• a housing defining a pressure chamber therewithin;
• said housing having an inlet communicated to a fluid pressure source and an outlet;
• valve means movable within said housing from a first position venting said pressure chamber and preventing communication from the inlet to the pressure chamber to a second position, said valve means communicating successively higher fluid pressure levels from said inlet into said pressure chamber as the valve means is shifted from said first position;
• an output piston slidably mounted in said pressure chamber and responsive to the fluid pressure communicated into the latter;
• operator-actuated means for operating said valve means;
• and
• means for releasing the fluid pressure level in the pressure chamber when the pressure level at the inlet is less than the pressure level in the pressure chamber;
• said last-mentioned means being check valve means controlling communication between said pressure chamber and the inlet;
• said check valve means permitting communication between the pressure chamber and the inlet whenever the pressure level in the pressure chamber exceeds the pressure level in the inlet, but prohibiting communication in the reverse direction;
• passage means extending through said valve means to communicate said inlet with said pressure chamber as the valve means is moved away from the first position, said passage means venting the pressure chamber when the valve means is in the first position and communication from said inlet through said passage means is prevented, said valve means controlling communication through said passage means to control the pressure level in the pressure chamber;
• said passage means including a branch communicated with said inlet independently of the position of said valve means, said check valve means being located in said branch.