What component in the vacuum brake booster ensures that vacuum does not escape the booster when the engine is turned off?

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Maintaining vacuum in the brake booster after the engine is turned off is the key idea. The component that keeps that vacuum from escaping is the check valve. It’s a one-way valve in the booster’s vacuum line that allows vacuum to flow into the booster when the engine is running, but closes to prevent air from leaking back out when the engine is shut off. This sealing action preserves the booster’s vacuum for a short time, so the brake pedal remains easier to press right after shutdown. The other parts don’t serve this sealing role: the atmospheric valve would vent to air and undo the vacuum hold, the return spring helps the piston return but doesn’t stop vacuum loss, and the term vacuum valve isn’t as precise as the one-way check valve.