Automobile & Light Truck Certification Test A5 BrakesHydraulic System Diagnosis and RepairMedium
A technician is performing a brake fluid flush on a vehicle. They notice that the new brake fluid being added from the bottle is slightly cloudy or milky in appearance. What is the MOST likely cause of this abnormal appearance?
- AThe fluid is contaminated with air from the bottling process.
- BThe brake fluid has absorbed excessive moisture.
- CThe fluid is incompatible with the existing system components.
- DThe fluid has been exposed to extreme cold temperatures.
Show answer & explanationAnswer & explanation
Correct answer: B. The brake fluid has absorbed excessive moisture.
Brake fluid, especially DOT 3/4/5.1, is hygroscopic, meaning it readily absorbs moisture. Absorbed moisture can cause the fluid to appear cloudy or milky and significantly lowers its boiling point.
Why the other options are wrong
- A. While some air might be present, it typically dissipates quickly and wouldn't cause a persistent cloudy/milky appearance in a sealed bottle.
- C. Incompatibility would likely lead to sludge or separation, not necessarily a milky appearance from the bottle itself.
- D. Extreme cold might increase viscosity or cause some components to separate in certain fluids, but a milky appearance is more indicative of water contamination.
Brake Fluid Moisture Contamination
Brake fluid (glycol-based) is hygroscopic and absorbs moisture from the air, which can cause it to appear cloudy or milky and significantly reduce its boiling point.
- Hygroscopic nature.
- Milky/cloudy appearance indicates water.
- Lowers boiling point, leading to vapor lock.
Memory trick: Cloudy fluid? Water's in the brew, making trouble for you.