What are the differences between DOT3 and DOT4 brake fluids?
2 Answers
Here are the differences between DOT3 and DOT4 brake fluids: 1. DOT3: Generally alcohol-ether type. The chemical composition of alcohol-ether type is low-polyethylene glycol or propylene glycol. Low-polyethylene glycol or propylene glycol has strong hydrophilicity, so its water content will gradually increase during use or storage. Since the boiling point of brake fluid decreases as the water content increases, its braking performance will decline accordingly. When you find that you need to step on the brake pedal harder to brake, a very likely reason is that the water content of the brake fluid is too high. Brake fluid is generally replaced every two years. 2. DOT4: Generally ester type. The ester type is based on the alcohol-ether type with the addition of a large amount of borate ester. Borate ester is formed by the esterification reaction of low-polyethylene glycol or propylene glycol with boric acid. Borate ester has a higher boiling point than low-polyethylene glycol or propylene glycol, so its braking performance is better. Borate ester also has strong moisture resistance, which can decompose the absorbed water, thereby slowing down the decrease in boiling point caused by water absorption. Therefore, the ester type has better performance than the alcohol-ether type and is also more expensive.
As a seasoned car enthusiast with years of experience, I want to talk about the differences between DOT3 and DOT4 brake fluids. Back when I first started tinkering with cars, I used DOT3 brake fluid, and during long-distance mountain drives, the brakes always felt mushy. After extensive troubleshooting, I discovered it was a high-temperature issue—DOT3 has a significantly lower boiling point than DOT4, meaning it’s prone to forming bubbles during aggressive driving or on steep slopes, leading to weaker braking performance, which is dangerous. Later, when I switched to DOT4, the improvement was noticeable: faster brake response and greater reliability, thanks to DOT4’s higher boiling point, which can handle more demanding conditions. Beyond boiling points, another major issue is moisture absorption: DOT3 absorbs water from the air more easily, which over time can cause corrosion or degraded performance in the brake system. DOT4 is designed better, absorbing moisture much slower, making maintenance easier. I advise against mixing them—while DOT4 is compatible with DOT3 systems, the reverse can cause problems. It’s best to consult your car’s manual to choose the right type. In short, upgrading to DOT4 adds an extra layer of safety for daily driving, especially if you enjoy spirited driving or tackling challenging road conditions.
