What should happen to the brake pedal during the vacuum booster test when starting the engine?

During the vacuum booster test, when starting the engine, the brake pedal is expected to move down. This occurs because the brake booster, which relies on engine vacuum to amplify the force applied to the brake pedal, becomes activated once the engine starts. The increase in vacuum assists the driver in applying brake force with less effort.

When the engine is turned on, the vacuum system generates a pressure differential that creates more force on the brake pedal. Therefore, the pedal will naturally move downward as the booster engages and effectively helps to push the piston(s) within the brake system, enhancing braking performance. This pedal movement is a clear and expected sign that the brake booster is functioning correctly.

If the brake pedal were to move up, remain still, or vibrate, these outcomes would suggest issues with the brake booster or the braking system, indicating that it might not be operating properly. Hence, the response of the pedal moving down confirms that the system is working as designed.