What is a critical factor to consider when changing brake fluid?

When changing brake fluid, moisture content is a critical factor to consider because brake fluid is hygroscopic, meaning it absorbs moisture from the air over time. This moisture can significantly lower the boiling point of the brake fluid, which can lead to vapor lock and decreased braking performance, especially under high-stress conditions such as during heavy braking or when driving in hilly areas. Additionally, the presence of moisture can lead to corrosion within the brake system components, potentially resulting in brake failure or diminished effectiveness.

While ambient temperature, the color of the brake fluid, and the type of vehicle may offer insights or additional considerations, they do not have as direct an impact on the performance and safety implications of the brake system as moisture content does. Therefore, monitoring and addressing the moisture content in brake fluid is essential for maintaining optimal braking performance and ensuring the safety of the vehicle.