When testing a hydraulic brake with power assist, what should happen when you turn the engine on?

Answer: C — The pedal should drop slightly

When testing a hydraulic brake with power assist, when you turn the engine on, the pedal should drop slightly. This is because the power assist uses engine-generated vacuum to assist with brake pedal pressure. As the engine turns on, the vacuum produced causes a slight drop in the pedal as the brake system is activated. Options A and B are incorrect because the pedal should not remain static or lift when the engine is turned on. Option D is incorrect because the pedal should not vibrate when the engine is turned on.