Can DOT4 Replace DOT3?
3 Answers
DOT4 cannot replace DOT3. Brake fluids of different types and brands should not be mixed. For braking systems with special requirements, a specific type of brake fluid must be used. Since the formulations of brake fluids vary by brand and type, mixing them can degrade the performance of the brake fluid. The differences between DOT4 and DOT3 are as follows: 1. Performance: DOT4 performs better than DOT3, especially in high-temperature conditions. 2. Boiling point: The equilibrium reflux boiling point of DOT4 is 230°C, while that of DOT3 is 205°C. 3. Usage characteristics: Cars using DOT4 brake fluid can brake more frequently than those using DOT3 brake fluid.
It can be used as a temporary substitute, but long-term mixing is not recommended. I've encountered many car owners during repairs who thought they could just add any type, only to end up with a spongy brake pedal. DOT4 has a boiling point 30-40 degrees higher than DOT3, making it more stable at high temperatures, but it absorbs water like crazy. If you have half a reservoir of DOT3 left in your car and pour in DOT4 directly, the boiling point of the mixed fluid might be lowered by the old fluid, making vapor lock more likely during continuous downhill braking. If you really want to switch, you need to completely drain the old fluid, even purging the brake caliper's oil. Remember, don't mix different brands either—additives can react and corrode rubber seals.
Veteran drivers advise caution when operating. My old pickup originally required DOT3 fluid, but once during a long trip when DOT3 wasn't available, I added DOT4 instead. After three months of driving, I noticed the clutch slave cylinder was leaking. The mechanic said the rubber seals had corroded. Turns out DOT4 has higher borate ester content, which is more aggressive on older vehicle's rubber components. Now I always check the manual when changing fluids - especially important for pre-2005 vehicles. However, mixing small amounts (half a bottle) in emergencies is acceptable, but complete replacement is necessary when you get home. Remember to bleed the system when changing fluid - soft brake pedals from air in the system can be extremely dangerous.
