What is the difference between DOT3 and DOT4 brake fluid?

[MacMicah]

DOT4 performs better than DOT3, mainly in terms of high-temperature performance. 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 details: DOT3 is generally alcohol-ether-based. The chemical components of alcohol-ether-based brake fluid are low-polymer ethylene glycol or propylene glycol. These compounds have strong hygroscopicity, so their water content gradually increases during use or storage. Since the boiling point of brake fluid decreases as water content increases, its braking performance declines accordingly. If you find yourself needing to press the brake pedal harder to stop, a likely reason is excessive water content in the brake fluid. Brake fluid is typically replaced every two years. DOT4 is generally ester-based. Ester-based brake fluid is made by adding a large amount of borate ester to alcohol-ether-based fluid. Borate ester is formed through the esterification reaction of low-polymer ethylene glycol or propylene glycol with boric acid. Borate ester has a higher boiling point than low-polymer ethylene glycol or propylene glycol, resulting in better braking performance. Borate ester also has strong moisture resistance, as it can decompose absorbed water, slowing the decrease in boiling point caused by water absorption. Therefore, ester-based brake fluid performs better than alcohol-ether-based fluid but is also more expensive. According to GB12981-2003 "Motor Vehicle Brake Fluid," brake fluids are classified into HZY3, HZY4, and HZY5, corresponding to DOT3, DOT4, and DOT5 internationally. Precautions for using automotive brake fluid: If gasoline, diesel, or engine oil accidentally mixes into a brake system using synthetic brake fluid, the incompatibility between the fluids can reduce braking effectiveness. After driving 40,000 kilometers or using brake fluid continuously for over two years, the fluid may degrade due to prolonged use, so timely replacement is essential. For vehicles equipped with a brake fluid level warning device, regularly check if the warning light is flashing and if the sensor is functioning properly. If the brake fluid is low, replenish it promptly. Stored brake fluid should be kept between the minimum and maximum marked levels. If braking feels inconsistent during normal driving, replace the brake fluid 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, the brake fluid quality may be compromised. In this case, replace it with high-quality brake fluid and replace the rubber cup as well. During seasonal changes, especially in winter, if braking performance declines, the brake fluid grade may not suit the winter climate. When replacing the 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 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. If water contaminates the brake fluid or if impurities or sediment are present, replace or thoroughly filter the fluid immediately. Otherwise, it may lead to insufficient braking pressure and reduced effectiveness. Replace brake fluid periodically: The volume of brake fluid changes with temperature, so the reservoir has a vent hole. Air drawn in through this hole may contain moisture or impurities, which the brake fluid absorbs or dissolves, lowering its boiling point. During prolonged braking, the braking system heats up, increasing the risk of vapor lock. Compressed air can reduce braking force or cause failure. For safety, brake fluid must be replaced regularly.

[DelJoshua]

I've been driving the old family car for many years and know how important brake fluid is. The main difference between DOT 3 and DOT 4 lies in their boiling points. DOT 4 has a wet boiling point over 20 degrees higher than DOT 3, meaning when the fluid contains moisture, DOT 4 is less likely to boil and vaporize under high temperatures. This is crucial because during long downhill descents or aggressive driving, brake temperatures can soar. If the fluid boils, the brake pedal will feel spongy, and there's even a risk of brake failure. That's why I recommend upgrading to DOT 4, especially since most modern cars use DOT 4 now. Regularly check the fluid's color and moisture content - if it turns black or shows bubbles, replace it immediately. Generally, change it every two years - safety comes first.