What condition might cause the brake pedal to move down slightly when the engine is started in a vehicle with vacuum-assisted power brakes?

• A. A leaking power brake booster diaphragm
• B. A faulty brake line
• C. A malfunctioning brake master cylinder
• D. A faulty power brake booster check valve

Answer: (A)

In a vehicle equipped with vacuum-assisted power brakes, the brake pedal moving down slightly upon engine start-up is primarily indicative of the vacuum power brake booster functioning as intended. When the engine starts, it generates vacuum, which assists the brake booster. If the diaphragm within the power brake booster is leaking, it compromises the booster’s ability to create the necessary vacuum pressure. As a result, the brake pedal can move down when the engine starts because the leakage allows for less support from the power assist, causing a reduction in resistance against the force the driver's foot applies on the pedal.

In contrast, issues with brake lines or the master cylinder typically manifest through different symptoms, such as fluid leaks or a spongy or unresponsive brake pedal, rather than a downward movement of the pedal when the engine starts. A faulty check valve in the power brake booster system would usually result in a loss of vacuum pressure when the engine is off, leading to a more significant change in pedal feel when attempting to brake rather than a subtle movement upon starting the engine.