Automobile & Light Truck Certification Test A5 BrakesHydraulic System Diagnosis and RepairEasy
A technician is performing a brake fluid flush on a vehicle that uses DOT 4 brake fluid. During the flush, they accidentally introduce a small amount of DOT 5.1 brake fluid into the system. What is the MOST immediate concern regarding this mixture?
- ANo significant concern, as DOT 5.1 is compatible with DOT 4.
- BReduced boiling point, increasing the risk of brake fade.
- CIncreased fluid viscosity, leading to sluggish brake response.
- DChemical incompatibility causing seal degradation and fluid contamination.
Show answer & explanationAnswer & explanation
Correct answer: A. No significant concern, as DOT 5.1 is compatible with DOT 4.
DOT 5.1 is a glycol-ether based fluid, similar to DOT 3 and DOT 4, and is fully compatible with DOT 3 and DOT 4 systems. It simply has a higher boiling point than DOT 4.
Why the other options are wrong
- B. DOT 5.1 has a higher boiling point than DOT 4; mixing would potentially raise, not reduce, the overall boiling point (though not significantly due to small amount).
- C. DOT 5.1 has similar viscosity to DOT 4, not increased.
- D. DOT 5.1 is glycol-based like DOT 3 and DOT 4, so there is no chemical incompatibility with seals. DOT 5 (silicone-based) is incompatible.
Brake Fluid Compatibility (DOT 4 & DOT 5.1)
DOT 4 and DOT 5.1 brake fluids are both glycol-ether based and are fully compatible with each other and with systems designed for DOT 3, 4, or 5.1.
- DOT 3, DOT 4, and DOT 5.1 are glycol-ether based.
- These fluids are hygroscopic and compatible with each other.
- DOT 5 is silicone-based and NOT compatible with other DOT fluids.
- Higher DOT numbers (4, 5.1) generally indicate higher boiling points.
Memory trick: Glycol fluids mix, but silicone stands alone.