What is the difference between DOT3 and DOT4 brake fluid?

[MacOwen]

The main difference between the two lies in their boiling points. DOT4 performs better than DOT3, especially in high-temperature conditions. The equilibrium reflux boiling point of DOT4 is 230°C, while that of DOT3 is 205°C. Vehicles using DOT4 brake fluid can brake more frequently. Additionally, their compositions differ. Below are the detailed explanations: DOT3 is typically glycol-ether based. The chemical components of glycol-ether are polyglycol or propylene glycol. Polyglycol or propylene glycol has strong hygroscopicity, so its water content gradually increases during use or storage. Since the boiling point of brake fluid decreases as water content rises, its braking performance declines accordingly. If you notice that you need to press the brake pedal harder to stop, a likely reason is that the brake fluid has excessive water content. DOT4 is generally ester-based. Ester-based brake fluid is made by adding a large amount of borate ester to glycol-ether. Borate ester is formed through the esterification reaction of polyglycol or propylene glycol with boric acid. Borate ester has a higher boiling point than polyglycol or propylene glycol, resulting in better braking performance. Borate ester also has strong moisture resistance, as it can decompose absorbed water, thereby slowing the decrease in boiling point caused by water absorption. Therefore, ester-based brake fluid performs better than glycol-ether-based brake fluid but is also more expensive. According to GB12981-2003 "Motor Vehicle Brake Fluid," brake fluids are classified as HZY3, HZY4, and HZY5, corresponding to DOT3, DOT4, and DOT5 internationally. Precautions for using automotive brake fluid: If gasoline, diesel, or engine oil is accidentally mixed into a synthetic brake fluid system, the incompatibility between the fluids can reduce braking effectiveness. For vehicles that have traveled 40,000 kilometers or have used brake fluid continuously for over two years, the brake fluid may deteriorate due to prolonged use, so timely replacement is essential. For vehicles equipped with a brake fluid level warning device, always monitor whether the warning light is flashing and whether the sensor is functioning properly. When the brake fluid is low, it should be replenished promptly. The stored brake fluid should be kept between the marked minimum and maximum capacity levels. If the braking feels inconsistent during normal driving, the brake fluid should be replaced immediately. Before replacement, clean the braking system with alcohol. If the vehicle pulls to one side during braking, conduct a comprehensive inspection of the braking system. If the wheel cylinder rubber cup is excessively swollen, it may indicate poor-quality brake fluid. In this case, replace it with high-quality brake fluid and change the rubber cup as well. During seasonal changes, especially in winter, if braking performance declines, the brake fluid grade may not be suitable for winter conditions. When replacing the brake fluid, choose one with lower viscosity at low temperatures. Do not mix different types or brands of brake fluid. For braking systems with special requirements, use the specified brand of brake fluid. Since different brands and types of brake fluid have varying formulations, mixing them can degrade performance. Even if some brands claim compatibility or substitutability, their performance may still be unsatisfactory, so avoid long-term use of mixed brake fluids. If water is mixed into or absorbed by the brake fluid, or if impurities or sediments are found, replace or thoroughly filter the brake fluid immediately. Otherwise, it may lead to insufficient braking pressure and affect braking performance. Regularly replace brake fluid: The volume of brake fluid changes with temperature, so the brake fluid reservoir has a vent hole. The air drawn in through this hole contains moisture or impurities, which can be absorbed or dissolved by the brake fluid, lowering its boiling point. During prolonged braking, the temperature of the braking system rises, increasing the risk of vapor lock in the brake lines. Compressed air can reduce braking force or cause brake failure. For driving safety, brake fluid must be replaced regularly.

[DiHailey]

As someone who frequently drives on highways and has experience with many older cars, I'm quite familiar with brake fluid. Simply put, the biggest difference between T3 and T4 lies in their boiling points: T3, known as DOT 3, has a lower boiling point, around 205°C dry boiling point, which can lead to overheating during long downhill drives in summer, causing the brakes to feel spongy. T4, or DOT 4, has a higher boiling point of approximately 230°C, making it more suitable for aggressive driving or hot weather. I always recommend using T4 because it absorbs less moisture, meaning the boiling point drops more slowly when water gets in, maintaining brake responsiveness. Change it every two to three years, and avoid mixing different types as it can affect performance. For older cars, if the manual specifies T3, it's acceptable, but I've found T4 to perform noticeably better, especially in stop-and-go traffic with frequent braking. Honestly, changing brake fluid doesn't cost much—safety comes first, so don't skimp. Regularly check the fluid level and color, and replace it immediately if it appears cloudy or foamy, as this is crucial for driving safety.