When testing a vacuum power brake booster, what should happen to the brake pedal after pumping it and starting the engine?

When testing a vacuum power brake booster, pumping the brake pedal while the engine is off causes the pedal to feel firm initially due to the lack of vacuum assistance. However, when the engine is started, the vacuum is applied to the brake booster, which enhances the braking force. As a result, the brake pedal should drop slightly, indicating that the booster is functioning correctly by providing additional assistance.

This slight drop should be accompanied by a firm feeling in the pedal, signaling that the brake system is responding as expected and that the booster is properly assisting with the braking effort. A drop without excessive sinking indicates a healthy vacuum boost action, helping to confirm that the brake booster is operating effectively.