Final answer:
The wheel cylinder is the part of the drum brake assembly filled with brake fluid, which helps push the brake shoes against the drum to stop the vehicle.
Step-by-step explanation:
The part of the drum brake assembly that is filled with brake fluid is the wheel cylinder. When you press the brake pedal, the master cylinder forces brake fluid through the brake lines into the wheel cylinder. As the wheel cylinder receives the brake fluid, it pushes the brake shoes outward so they can contact the drum, creating the friction needed to stop the vehicle. The parts of the drum brake assembly such as the anchor pin, master cylinder, and shoe have different functions. The anchor pin holds the brake shoes in place, the master cylinder supplies the brake fluid, and the brake shoe is the component that actually contacts the brake drum to stop the vehicle.