What is the Difference Between DOT4 and DOT3 Brake Fluid?

[SiennaLynn]

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, as detailed below: DOT3 is typically glycol-ether-based. The chemical components of glycol-ether brake fluid 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 find that you need to press the brake pedal harder to stop, a likely reason is that the brake fluid has excessive water content. Brake fluid should generally be replaced every two years. DOT4 is typically ester-based. Ester-based brake fluid is made by adding a large amount of borate ester to the glycol-ether base. 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 decline 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 into 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 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 degrade 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 brake fluid is low, it should be replenished promptly. Stored brake fluid should be kept between the marked minimum and maximum capacity levels. If braking becomes 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 simultaneously. During seasonal changes, especially in winter, if braking performance declines, the brake fluid grade may not suit winter conditions. When replacing 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 a specific brand of brake fluid. Since different brands and types of brake fluid have varying formulations, mixing them can degrade performance. Even brands labeled as compatible or interchangeable may not perform satisfactorily, so avoid long-term mixed use. If water contaminates the brake fluid or if impurities or sediment are found, replace or thoroughly filter the fluid immediately. Otherwise, it may lead to insufficient braking pressure and reduced effectiveness. Regularly replace brake fluid: 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 can be absorbed or dissolved by the brake fluid, 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 periodically.

[Melanie]

As a veteran driver with twenty years of experience, I know the difference between DOT3 and DOT4 brake fluid all too well. DOT3 is the older standard, with a boiling point of only around 400 degrees. When driving on mountain roads in summer or braking frequently, it tends to overheat and bubble, making the brake feel spongy. DOT4 has a boiling point close to 450 degrees, offering better heat resistance, more stability during hard braking or heavy loads, and much quicker brake response. Additionally, DOT3 absorbs water easily, which can compromise performance in humid weather, requiring more frequent changes. DOT4 has lower water absorption, a longer service life, and saves some maintenance effort. If DOT3 and DOT4 are accidentally mixed during a fluid change, the fluid can degrade and reduce braking effectiveness—new drivers should never try this. I recommend using DOT4 regularly, especially for older cars or long-distance driving, as safety comes first. In short, DOT4 is like an upgraded version—more expensive but worth it.