Why Does Brake Fluid Contain Moisture?
4 Answers
Brake fluid has an important characteristic of being hygroscopic, meaning it can completely blend with water. Here are the relevant details: 1. Brake Fluid: Brake fluid is a type of oil in the car's braking system, primarily serving to transmit braking force. 2. Braking System: A car's braking system consists of a vacuum booster, master cylinder, wheel cylinder, brake pads, and brake discs. When the brake pedal is pressed, the vacuum booster provides assistance, making it easier for the driver to press the pedal without much effort. 3. Types of Brake Fluid: Based on composition, brake fluid is categorized into synthetic and mineral types, among others.
My wife and I went on a road trip and noticed the brakes felt a bit soft. After inspection, we found out the brake fluid had water contamination. Why does this happen? Actually, brake fluid is inherently hygroscopic, meaning it easily absorbs moisture from the air. During normal driving, especially in humid weather or when parked outdoors, moisture can gradually seep into the brake system. The main entry points are a loose master cylinder cap or poorly sealed brake line connections, allowing air to enter and condense into water inside. According to my research, DOT4-type fluid has even stronger moisture absorption and degrades faster. Excessive water content lowers the boiling point, causing bubbles to form during braking, leading to reduced stopping power and potentially serious accidents. Last year, I had the fluid replaced, and the mechanic recommended changing it every two years or 20,000 kilometers to keep the system dry. Daily precautions like tightening caps properly and avoiding prolonged exposure to air can extend its lifespan. This knowledge is quite useful, so I’m sharing it to remind everyone to drive safely.
With over 20 years of experience in auto repair, I've seen countless cases of brake fluid exceeding the water content limit. The root cause lies in brake fluid's natural hygroscopic property—it actively absorbs moisture from the air, much like a sponge soaking up water. This is especially pronounced during humid seasons, such as rainy periods or in coastal cities, where high air moisture levels make older vehicles with poor seals more susceptible to infiltration. When testing with a moisture detection pen, over-saturated fluid shows excessive bubbling. Water contamination is severely damaging: not only does it lower the boiling point, causing vapor lock and brake failure, but it also corrodes metal components like brake calipers and lines, driving up repair costs. I advise car owners to regularly visit shops for fluid quality checks. For standard DOT4 fluid, a replacement every two years is ideal. If cloudiness or layering is observed, immediate action is required. Opting for high-quality fluid can slow moisture absorption, ensuring driving safety—never take this lightly.
Safety first, as a driver I must remind you: the issue of brake fluid moisture contamination should not be underestimated. Moisture infiltration mainly occurs due to poor system sealing, where humidity enters through the cap or pipe openings and condenses when cold. Daily parking in humid environments makes this prone to happen. This lowers the boiling point of brake fluid, causing it to boil during high-temperature driving, resulting in a heavy brake pedal and slow response, increasing the risk of rear-end collisions. It may also corrode components, shortening their lifespan. Everyone should pay attention to the sealing of the oil tank cap during car maintenance, regularly check and replace the fluid – typically every two years to prevent potential hazards. Address any issues early; don't risk major dangers to save small amounts of money.
