What Type of Oil is Brake Fluid?
2 Answers
According to the American Petroleum Institute regulations, brake fluids are categorized into three types: DOT3, DOT4, and DOT5.1. Below is an introduction to the relevant content: 1. DOT3: DOT3 is generally an alcohol-ether type brake fluid (polyglycol or propylene glycol). 2. DOT4 and DOT5: DOT4 and DOT5 are ester-based (with large amounts of borate esters added to the alcohol-ether base) and silicone-based (refined from high-cost silicone base oils). 3. DOT5.1: DOT5.1 is specifically for racing or military use, so it should not be confused with DOT5, otherwise incompatibility issues may arise.
Last time I helped my neighbor fix his old Jetta, he asked the same question. Brake fluid is actually a type of hydraulic oil, mainly used to transmit the force from the brake pedal. The common types we see are DOT3 and DOT4, which are clearly labeled on the bottle. This stuff is most afraid of water absorption—once it absorbs moisture, its boiling point drops sharply. When you press the brake, the brake pads can heat up to 200-300 degrees Celsius. If the brake fluid contains water and boils, it creates bubbles in the lines, making the pedal feel spongy and ineffective. Mechanics often use a test pen to check the fluid in the reservoir—if the moisture content reading exceeds 3%, it must be replaced.
