What distinguishes DOT 3 and 4 brake fluid from DOT 5 brake fluid?

The distinction between DOT 3 and 4 brake fluid compared to DOT 5 brake fluid primarily lies in their hygroscopic properties. DOT 3 and 4 fluids are hygroscopic, meaning they absorb moisture from the environment over time. This characteristic is significant because as they absorb water, their boiling point decreases. This absorption can lead to vapor lock in the braking system, where the formation of steam from the heated water in the fluid can compromise braking performance.

Conversely, DOT 5 is a silicone-based fluid that is non-hygroscopic. It does not absorb moisture, which makes it resistant to water-related boiling issues; however, this also means that any moisture present can settle in the system and potentially cause corrosion over time.

Understanding the hygroscopic nature of brake fluids helps technicians make informed choices when servicing brake systems, ensuring optimal performance and safety.