Which of the following components is NOT part of the hydraulic system that must be free of leaks and structural rust?

In hydraulic brake systems, components must be maintained in optimal condition to ensure safe and effective performance. The master cylinder, brake lines and hoses, and calipers are all integral parts of this system.

The master cylinder is critical as it generates hydraulic pressure when the brake pedal is pressed, enabling the brakes to function. Brake lines and hoses are responsible for transmitting that hydraulic pressure to the brake calipers. The calipers then clamp down on the brake pads, creating the friction necessary to slow or stop the vehicle. Any leaks or structural rust in these parts could lead to brake failure, posing a significant safety risk.

In contrast, the fuel line is part of the vehicle's fuel system, not the hydraulic brake system. Ensuring the fuel line is in good condition is important for the vehicle’s operation, but it does not directly relate to the hydraulic system involved in braking. Therefore, the fuel line is the component that does not need to be checked for leaks and structural rust in this context, making it the correct answer to the question.