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

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In a vacuum brake booster, two one-way valves control the pressure on either side of the diaphragm to keep or release boost as needed. When the pedal is in the hold position, neither path is allowed to flow, so both control valves stay closed. This seals the two chambers, preserving the existing vacuum on the side connected to the engine and keeping the other side from changing, which prevents any loss of assist while the system is idle. If either valve were open, vacuum would leak away or air would enter, reducing the booster’s readiness to provide assistance on the next pedal press.