What is the boiling point comparison between DOT 5 and DOT 4 brake fluid according to Technician A?

• A. DOT 5 has a higher boiling point than DOT 4
• B. DOT 5 has a lower boiling point than DOT 4
• C. Both have the same boiling point
• D. DOT 4 is water-based

Answer: (B)

DOT 5 brake fluid, which is silicone-based, indeed has a lower boiling point compared to DOT 4 brake fluid, which is glycol-based. The boiling point of DOT 5 is generally around 260°C (500°F), while DOT 4 has a higher boiling point, typically around 230°C (446°F) for the standard version and even higher for the "DOT 4 LV" (Low Viscosity) version.

This lower boiling point of DOT 5 makes it less suitable for high-performance vehicles or applications where braking temperatures can be exceptionally high, as it may lead to vapor lock, which can compromise braking effectiveness. Understanding these properties is crucial for technicians and mechanics when selecting the appropriate brake fluid for various automotive applications, ensuring safety and performance standards are met.