What is the difference between DOT brake fluid and mineral oil?
Avid, Formula, Hayes and Hope use DOT fluid and Magura and Shimano use mineral oil. You should never use DOT brake fluid in a system designed to use mineral oil and vice versa.
DOT brake fluids are hygroscopic, they absorb water from the air over time. Mineral oil, on the other hand, does not. Mineral oil and water will repel each other and not mix, therefore any introduction of water to a mineral oil filled system will cause the water to pool and can freeze in cold temperatures causing brake failure. So brake sistems using mineral oil can have problem in extreme low temperatures.
Unlike DOT fluids, mineral oil is non-corrosive and will not harm your skin or you bikes paint finish if spilled. However DOT brake fluid has a higher boiling point than mineral oil and will therefore outperform mineral oil under extreme use.
What is the difference between DOT 3/4/5/5.1 brake fluids?
DOT 3, 4 & 5.1 brake fluid is glycol-based. The main differences in these three DOT fluids are their boiling temperatures. DOT 4 has a higher boiling point than DOT 3, and DOT 5.1 has the highest boiling point of all.
DOT 5 fluid (not 5.1) is a silicone based fluid and is incompatible with all other DOT fluids. DOT 5 fluid should never be used in your hydraulic mountain bike brakes.
How does air and moisture intake reduce the braking performance?Air is a gas, gases are compressible whereas fluids are pretty much incompressible. If there is air within the brake system some of the pressure you exert at the lever will be wasted as the air is compressed; under such circumstances your brake lever can feel 'spongy' and non consistent. DOT brake fluid is hygroscopic, which means it absorbs water from the atmosphere at normal humidity levels over a period of time. This water contaminated brake fluid will have a lower overall boiling point than fresh DOT brake fluid. As we know brake fluid can become very hot under heavy use and can eventually 'boil'.