What is the difference between DOT4 and DOT3 brake fluid?

[DelStephanie]

The main difference between the two lies in their boiling points. DOT4 performs better than DOT3, particularly 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 hygroscopic properties, meaning its water content gradually increases during use or storage. Since the boiling point of brake fluid decreases as water content rises, its braking performance diminishes 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 formulated by adding a significant amount of borate ester to the glycol-ether base. Borate ester is produced 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 exhibits 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 and 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. For vehicles driven normally for 40,000 kilometers or with brake fluid used 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, replenish it promptly. Stored brake fluid should be maintained between the marked minimum and maximum capacity levels. If braking becomes inconsistent during normal driving, replace the brake fluid immediately. Before replacement, clean the brake system thoroughly with alcohol. If the vehicle pulls to one side during braking, conduct a comprehensive inspection of the brake 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 brake 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 certain brands claim compatibility or substitutability, their performance may still be unsatisfactory, so avoid long-term use of mixed fluids. If water contaminates the brake fluid or if impurities or sediment are detected, replace or filter the fluid promptly. Otherwise, insufficient braking pressure may occur, affecting braking performance. Regularly replace brake fluid: The volume of brake fluid changes with temperature variations. The brake fluid reservoir has a vent hole, and the air drawn in through it may contain moisture or impurities. Moisture can be absorbed or dissolved by the brake fluid, lowering its boiling point. During prolonged braking, the brake system heats up, increasing the risk of vapor lock in the brake lines. Compressed air can lead to reduced braking force or failure. For driving safety, brake fluid must be replaced periodically.