What is the difference between good and bad brake fluid?
3 Answers
Brake fluid quality can be distinguished by its grade. According to the international DOT classification standard, theoretically, the worst should be DOT3, followed by DOT4, and the better one should be DOT5. However, in reality, considering practical conditions, the best is DOT5.1 brake fluid, followed by DOT4 brake fluid, and finally DOT3 brake fluid. The three most important indicators for identifying the authenticity and quality of brake fluid are boiling point, low-temperature fluidity, and corrosiveness. Based on these indicators, the quality of brake fluid can be distinguished as follows: 1. Poor-quality brake fluid has poor high-temperature resistance: Under high-temperature conditions, it is prone to vaporization, forming bubbles that are easily visible to the naked eye, creating air resistance in the brake system pipelines and hindering the flow of brake fluid; 2. Corrosiveness test: Brake fluid mixed with methanol or mixed alcohols and water has strong corrosiveness to metal objects. Placing a metal object in it to observe whether it rusts can help identify its quality; 3. Fluidity test: Based on the kinematic viscosity characteristic of brake fluid, which is its fluidity at low temperatures. In extremely cold northern regions, qualified brake fluid can still flow normally, while poor-quality brake fluid will show varying degrees of solidification. Brake fluid in this state basically loses its braking performance, and once the brake fluid freezes and expands, it can burst the brake lines, leading to more severe consequences.
Having driven for over a decade, the biggest difference between good and bad brake fluid lies in safety impact. High-quality brake fluid has a high boiling point, appears clear like vegetable oil when new, and can withstand the high temperatures generated during emergency braking without vaporizing and causing a spongy brake pedal. Inferior fluid turns yellowish with flocculent particles after absorbing moisture, drastically lowering its boiling point—sometimes below 150°C when water content is high. This makes it prone to vaporization during braking, resulting in a soft, unresponsive pedal that feels like stepping on a sponge, potentially increasing stopping distance and creating serious hazards. I regularly check my brake fluid using a moisture test pen; if water content exceeds 3%, I replace it immediately—no waiting for failure. Standard DOT4 fluid typically lasts about two years before it must be replaced to avoid corroding brake caliper components, as rust and leaks can lead to even bigger headaches.
As a long-time car repair enthusiast, I can tell the difference between good and bad brake fluid from daily maintenance. Good fluid has the right viscosity and flows smoothly, providing quick brake system response when replaced, with a firm and steady feel when you press the pedal. Bad fluid is either too thin or too thick, causing squeaky brakes or excessive pedal travel, which indicates aging. If the fluid in the reservoir turns black or bubbles, it means it has absorbed moisture and contains impurities—clear signs of deterioration. I check the fluid color every six months by opening the cap; clear and transparent is good, while cloudy or with sediment means it's time for an immediate change. When buying, opt for reputable brands like DOT 5.1 synthetic fluid, which resists high temperatures and corrosion. Don’t skimp on the cost—spending a couple hundred bucks on a fluid change now can save you thousands on major repairs later.
