Can a car with DOT3 use DOT4?
3 Answers
DOT3 and DOT4 brake fluids cannot be mixed. Here are some related details about brake fluid: 1. DOT3 and DOT4 brake fluids cannot be mixed, as doing so would lower the dry and wet boiling points of the brake fluid. This not only increases the chance of system vaporization causing vapor lock, leading to delayed or even failed braking, but also affects the stability of the brake fluid, posing certain risks during continuous braking. 2. Brake fluid, also known as hydraulic brake fluid, serves as a functional liquid in a car's hydraulic braking system to transmit pressure, enabling the wheel brakes to function. Most modern cars use hydraulic braking systems, where the brake fluid acts as the liquid medium to transmit braking force throughout the system. The working pressure of brake fluid is typically 2MPa, with some systems reaching 4-5MPa. Since liquid is difficult to compress, when the brake pedal is pressed, the pressure is transmitted to the master cylinder and then through the brake fluid to the wheel cylinders, generating significant braking force.
My old car originally used DOT3, and I switched to DOT4 last year. The operation wasn’t difficult—the key was having the repair shop completely drain the old fluid before adding the new one. Mixing the two is absolutely not an option, as blending them can lower the boiling point, potentially causing a spongy brake feel during high-speed braking. DOT4 performs more stably under high temperatures, especially when driving on mountain roads in summer or during long-distance continuous braking, where the braking feels noticeably firmer. However, be aware that DOT4 is more hygroscopic; instead of changing it every three years, now it’s necessary to check the moisture content every two years. After the change, it’s best to have the mechanic pump the brakes a few times to bleed the air and also inspect the brake pad wear. Overall, the upgrade is worthwhile—it enhances safety while remaining compatible with the original vehicle system.
No worries, I upgraded my off-roader from DOT3 to DOT4. The key points are: 1) Complete fluid flush is crucial - residual old fluid lowers the new fluid's boiling point; 2) Always choose reputable brands, never cut corners with knockoffs. DOT4 clearly outperforms in vapor lock resistance, especially during downhill braking with heavy loads when fluid temps spike above 200°C - it maintains consistent braking power. Remember to test pedal travel post-change; properly done, you'll feel firmer pedal feedback. Also, DOT4's rubber compatibility matches DOT3, so older vehicles' seals require no additional replacement - a real convenience factor.
