There's two major types of "brake fluid" used in hydraulic bicycle brakes:
- Mineral oil: Magura, Shimano;
- DOT 4/5.1 brake fluid: Avid, Hayes, Hygia.
DOT brake fluid
DOT brake fluid is used in car and motorcycle hydraulic brake systems. There are several different types of fluid:
- Dot 3, 4 and 5.1 are glycol-based fluids - glycol-based brake fluids are "hygroscopic" which means that they will absorb water.
- Dot 5 brake fluids are silicon-based and hydrophobic (they do not absorb water).
Most bicycle hydraulic systems use DOT 4 or 5.1.
Glycol based brake fluids are corrosive and can damage paint and other surfaces if not washed off immediately. You should avoid getting it on your hands as it can be a skin irritant.
What's the problem with brake fluid absorbing water?
There are two main problems when glycol based brake fluids absorb water:
- the boiling point of the fluid drops rapidly - as little as a 2% absorbed water can result in a 30% reduction in boiling point;
- absorbed water in the brake fluid can corrode internal parts of the brake system.
Why is the boiling point of brake fluid important?
The boiling point of brake fluid is all important to the proper function of hydraulic brakes. If the fluid boils then it is no longer a liquid that properly resists compression. Brakes will feel spongy and lack power until the brake fluid drops below its boiling point.
Can I used DOT 3 brake fluid in an emergency?
Yes, provided your brake system uses DOT 4 or 5.1 in the first place. DOT 3 has a lower boiling point than DOT 4 or 5.1.
Can I use DOT brake fluid in mineral oil systems?
No, glycol-based brake fluids if used in mineral oil systems will cause the seals to swell.
Magura claims that using mineral oil results in a virtually maintenance free brake system because unlike glycol based brake fluids, mineral oil does not absorb water. However, DOT 4/5.1 fluids regularly changed (yearly if possible) will offer superior boiling points and thereby performance under extreme braking conditions.
If the heat in your braking system never approaches the boiling point of the brake fluid then it doesn't matter either way.