What is a primary indication that a brake fluid is contaminated?

A primary indication that brake fluid is contaminated is discoloration. Brake fluid is typically a clear or light amber liquid when it's clean and in good condition. As brake fluid absorbs moisture over time and becomes contaminated, it can change color, often appearing darker or cloudy. This discoloration is a visual cue that indicates that the fluid may no longer provide adequate lubrication or protection for the braking system, which can lead to corrosion of internal components and decreased braking performance.

The other options, while they can be important in assessing brake system health, do not directly indicate contamination. Low fluid levels could be a sign of leakage or wear in the brake system but do not specifically point to contamination. An unusual smell could indicate other issues, such as overheating of the brake components, but doesn't provide a clear sign of fluid contamination. Increased pressure resistance is more related to the brake system's hydraulic dynamics rather than the condition of the fluid itself, and it doesn’t indicate whether the brake fluid has become contaminated.