What is the difference between T3 and T4 brake fluid?
2 Answers
The differences between brake fluid T3 and T4 are as follows: Different boiling points, different main ingredients, different water absorption characteristics, and different applicable ranges. DOT3 is generally of the glycol-ether type. The chemical composition of the glycol-ether type is low-polymer ethylene glycol or propylene glycol. DOT4 is generally of the ester type. The ester type is based on the glycol-ether type with the addition of a large amount of borate esters. Different boiling points: The equilibrium reflux boiling point of DOT3 is 205 degrees. The equilibrium reflux boiling point of DOT4 is 230 degrees. Different main ingredients: The main ingredient of DOT3 is glycol ether compounds. The main ingredient of DOT4 is polyglycol/borate/ester. Different water absorption characteristics: DOT3 is highly prone to water absorption. DOT4 is prone to water absorption. Different applicable ranges: The applicable range of DOT3 is 90km/h-140km/h. The applicable range of DOT4 is 140km/h-180km/h. DOT3 is generally of the glycol-ether type. The chemical composition of the glycol-ether type is low-polymer ethylene glycol or propylene glycol. Low-polymer ethylene glycol or propylene glycol has strong hydrophilicity, so its water content gradually increases 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 press the brake pedal harder to achieve braking, a likely reason is that the water content in the brake fluid is too high. Brake fluid should generally be replaced every two years. DOT4 is generally of the ester type. The ester type is based on the glycol-ether type with the addition of a large amount of borate esters. Borate esters are formed by the esterification reaction of low-polymer ethylene glycol or propylene glycol with boric acid. Borate esters have a higher boiling point than low-polymer ethylene glycol or propylene glycol, so they offer better braking performance. Borate esters also have strong moisture resistance, as they can decompose absorbed water, thereby slowing the decrease in boiling point caused by water absorption. Therefore, the ester type performs better than the glycol-ether type but is also more expensive. According to GB12981-2003 "Motor Vehicle Brake Fluid," brake fluids are classified into HZY3, HZY4, and HZY5, corresponding to the international standards DOT3, DOT4, and DOT5, respectively. 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 more than two years, the brake fluid may deteriorate due to prolonged use, so timely replacement is necessary. For vehicles equipped with a brake fluid level warning device, always check 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, a comprehensive inspection of the braking system should be conducted. If the wheel cylinder rubber cups are found to be excessively swollen, it may indicate poor-quality brake fluid. In this case, high-quality brake fluid should be selected for replacement, and the rubber cups should also be replaced. During seasonal changes, especially in winter, if braking performance declines, it may be due to the brake fluid's grade being unsuitable for winter conditions. When replacing the brake fluid, choose one with low 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 different formulations, mixing them can degrade performance. Even if some brands claim compatibility or substitutability, long-term use is not recommended. If water is mixed into the brake fluid or if impurities or sediments are found, the brake fluid should be replaced or thoroughly filtered 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. 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. When the car brakes for extended periods, the temperature of the braking system rises, and air bubbles may form in the brake lines, compressing the air and reducing braking effectiveness or causing failure. For driving safety, brake fluid must be replaced regularly.
As an average car owner, I've changed brake fluid a few times and noticed that T3 and T4 mainly differ in boiling point and moisture resistance. T3 has a lower boiling point and tends to absorb water in humid or rainy conditions, leading to spongy brakes or failure, which is especially dangerous during long-distance or mountain driving. T4 has a significantly higher boiling point, performs stably in both dry and wet conditions, and offers better moisture resistance, reducing the risk of corroding the brake lines. Never mix them, as their different formulations can react and form a gel-like substance that clogs the ABS system—I once mixed them by mistake and nearly had an accident. Always check your vehicle's manual; most modern cars require T4 and recommend changing it every two years for safety.
