What happens in the hold position of the vacuum brake booster?

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The hold position is about keeping the booster’s diaphragm isolated so the existing vacuum difference is preserved. When both the vacuum supply valve and the atmospheric/bleed valve are closed, no air can enter or escape from the booster chamber. This traps the current pressure on both sides of the diaphragm, maintaining the assist level for the next brake application and preventing loss of vacuum. If either valve were open, air could flow in or the vacuum could bleed off, reducing or changing the boost you feel next time you press the pedal.