What is the difference between DOT 3, DOT 4, DOT 5, and DOT 5.1 brake fluids?
DOT 3, DOT 4, and DOT 5.1 brake fluids are glycol-based compounds that are compatible with one another. On the other hand, DOT 5 brake fluid is silicone-based and should never be mixed with any other type of brake fluid. DOT 3, DOT 4, and DOT 5.1 brake fluids will damage painted surfaces. DOT 3 and DOT 4 fluids have lower boiling temperatures than DOT 5 and DOT 5.1. Furthermore, DOT 3, DOT 4, and DOT 5.1 fluids are hygroscopic--they absorb moisture from the air. This causes the fluid to turn dark, indicating that it is time for the brake fluid to be replaced. DOT 5 fluid will not damage paint, has a boiling temperature in excess of 500 degrees F, and is not hygroscopic.