How many control valves are in a vacuum brake booster?

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In a vacuum brake booster, airflow is controlled by a pair of valves. One is a vacuum supply check valve that keeps the booster’s vacuum from leaking back into the intake manifold when manifold vacuum drops or the engine isn’t supplying it. The other is a bleed or relief valve that opens when you press the brake pedal, allowing air to escape from the booster chamber to the atmosphere so the diaphragm can move and provide braking assistance. This combination gives the booster a stored vacuum for smooth, consistent assist and a controlled vent during braking. Having just one valve wouldn’t both hold vacuum and vent properly, and adding more valves would add unnecessary complexity, so two control valves is the correct arrangement.