What is a reason for failing a vacuum or hydraulic power booster?

The reason for failing a vacuum or hydraulic power booster is that there is no downward movement of the brake pedal when the engine starts. A properly functioning power booster should create a vacuum that assists in applying the brakes, leading to a noticeable change in the brake pedal position when the engine is running. If the pedal does not move down as expected, this indicates a failure in the booster’s ability to aid in brake application, which can compromise the effectiveness of the braking system and may lead to unsafe driving conditions.

In contrast, increased pedal travel might suggest other issues but does not directly indicate a failure of the booster itself. Excessive resistance in the pedal could point to mechanical issues unrelated to the booster, and noise during brake application may indicate other problems with the braking system or components but is not specifically related to the function of the power booster. Thus, the absence of pedal movement when starting the engine is a clear indicator that the booster is not functioning correctly and warrants failing the inspection.