What is the difference between DOT3 and DOT4 brake fluids?
2 Answers
The main differences between DOT3 and DOT4 lie in their boiling points and compositions. Below are the detailed explanations: 1. Boiling Point: The equilibrium reflux boiling point of DOT4 is 230°C, while DOT3 is 205°C. Additionally, the minimum wet boiling point of DOT4 is 144°C, compared to 140°C for DOT3. From these data, it is evident that DOT4 has higher boiling points than DOT3, which not only more effectively prevents vaporization in the system that could lead to vapor lock but also ensures stable braking performance during continuous braking. 2. Composition: DOT3 is typically glycol-ether based. The chemical components of glycol-ether are low-molecular-weight polyethylene glycol or propylene glycol, which have strong hydrophilic properties. DOT4, on the other hand, is generally ester-based. The ester type is formulated by adding a significant amount of borate esters to the glycol-ether base.
Having driven family cars for over a decade, I can explain that the main difference between DOT3 and DOT4 brake fluids lies in their boiling points. DOT4 typically has a dry boiling point around 230°C and a wet boiling point of approximately 155°C, while DOT3 has a dry boiling point of 205°C and a wet boiling point of 140°C. Higher boiling points mean the fluid is less likely to form bubbles during prolonged downhill driving or frequent braking, ensuring reliable brake system response. DOT4 also absorbs moisture more slowly, reducing the risk of corroding brake components. Although DOT4 is slightly more expensive, the enhanced safety is worth it. I use DOT4 in my vehicle, and regular fluid changes with boiling point checks help prevent unexpected failures. While DOT3 is acceptable for regular vehicles, upgrading to DOT4 provides extra peace of mind—especially when driving in mountainous areas or hot weather, where those extra few degrees in boiling point can be lifesaving.
