Vacuum boosters operate in three states or positions:

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Vacuum brake boosters use engine vacuum to multiply the driver’s braking effort, and they have three steady states: Released, Applied, and Holding. When released, the brake pedal is up and no pressure differential exists across the booster diaphragm, so no boost is provided. When you press the pedal, a valve exposes the rear chamber to atmosphere while the front chamber remains under vacuum, creating a pressure difference that multiplies the force applied to the master cylinder. If you hold the pedal in a fixed position, the booster maintains that pressure differential and continues to assist braking until you release the pedal.