Final answer:
To test hydraulic brakes with power assist, you must engage the parking brake and pump the brake pedal several times to build pressure; both steps are essential for safety and accurate testing.
Step-by-step explanation:
When preparing to test hydraulic brakes with power assist on a bus, you should first ensure that the vehicle is stationary by putting on the parking brake. This will prevent the bus from unintentionally moving while you are checking the brakes. Then, you should pump the brake pedal multiple times to ensure the power assist is correctly pressurizing the brake system. The idea is to build up enough pressure in the brake system to allow a proper assessment of the brake functionality. Both steps are essential before conducting tests on the hydraulic brakes, so the correct answer is to do all these things.
Hydraulic brakes rely on Pascal's principle, where force exerted on the brake pedal is increased by a lever and subsequently by the hydraulic system, equally distributing the pressure to the wheel cylinders. Proper function of this system is crucial for vehicle safety, and testing the assist mechanism is part of regular maintenance to ensure the pump that assists braking force is operating effectively.