Brake fluid is available in four grades: DOT 3, 4, 5, and 5.1. DOT 3, 4, and 5.1 fluids are glycol based and miscible, while DOT 5 fluid is silicone based and can’t be mixed with any other type of fluid. Glycol-based fluids are hydrophilic and will attract and absorb moisture out of the air. DOT 5 fluid is hydrophobic, but due to the repeated heating and cooling cycles and the imperfect sealing of master cylinders and calipers, all fluids will eventually ingest some quantity of water. The difference is that glycol-based fluids will pull moisture out of the air on their own while DOT 5 will not, meaning DOT 5 has a much longer service life.