Final answer:
Upon engine start, a hydraulic brake pedal with power assist should drop slightly, indicating that the power assist system has engaged.
Step-by-step explanation:
When testing a hydraulic brake system with power assist, and you turn the engine on, the pedal should drop slightly. This is because the power assist system, commonly a vacuum booster or a hydraulic/electric booster, begins to provide the additional force needed to help push the brake fluid through the system. If the pedal drops to the floor or drops halfway, this could indicate a problem with the brake system that would require further investigation.