Which type of brake fluid should not be mixed with others?

The correct answer is the type of brake fluid designated as DOT 5. This fluid is unique because it is silicone-based, unlike DOT 3 and DOT 4, which are glycol-based. The chemical differences between silicone and glycol-based fluids mean they do not mix well. When mixed, this can lead to a number of issues, including reduced effectiveness in braking performance and potential damage to the braking system components.

Using DOT 5 brake fluid requires the entire system to be flushed and filled with this fluid exclusively to ensure it functions correctly. Therefore, it is critical to avoid mixing DOT 5 with any other type, as doing so can compromise the entire braking system’s integrity. The other types of brake fluids, while still needing to be used with care, can technically be mixed, but it is not advisable for safety and performance reasons.