The control valves in a vacuum brake booster during the applied position will:

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A vacuum brake booster increases braking force by using a pressure difference across a diaphragm formed between an engine-vacuum side and an atmospheric side. Two control paths manage that difference: an atmospheric path (atmospheric valve) and a vacuum path (vacuum valve). In the applied position, atmospheric pressure is allowed to enter the pedal-side chamber while the vacuum-side valve is closed to isolate the engine vacuum. This creates a higher pressure on the pedal side relative to the vacuum side, pushing the diaphragm toward the master cylinder and providing boost. If both paths were open, pressures would quickly equalize and the boost would be lost; if both were closed, nothing would move. If the vacuum path remained open and the atmospheric path stayed closed, the boost would be reduced because atmospheric input wouldn’t be admitted.