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What is the right fluid to use in my hydraulic brakes?
I just ran in to this while replacing the hoses on my mountain bike. It's also a good idea to not do your own brake repair. Take that from me. It's not easy. It's best to stay with the manufacturers fluid, in case you need any service.
DOT Fluid
First, 'DOT' stands for the Federal Department Of Transportation. They set standards for brake fluid for cars based on boiling point and viscosity.
But first, what do the numbers mean? The DOT fluid rating is by their boiling point, not by composition.

Boiling point ranges

Dry boiling point Wet boiling point
DOT 3 205 °C (401 °F) 140 °C (284 °F)
DOT 4 230 °C (446 °F) 155 °C (311 °F)
DOT 5 260 °C (500 °F) 180 °C (356 °F)
DOT 5.1 260 °C (500 °F) 180 °C (356 °F)
Wet refers to fluid contaminated with water.

There are several different types of DOT brake fluid. Most are compatible, but there is an important incompatibility that you need to know. You might think that DOT 5.1 would be a better type of DOT 5, but it is not. Under NO CIRCUMSTANCES should DOT 5 and DOT 5.1 fluid be mixed.

Most types of brake fluid are based on a polyethylene glycol formula, but DOT 5 is based on a silicone base. DOT 5 and DOT 5.1 fluids have the same boiling point, yet they are completely incompatible, and you will void your warranty and have unreliable braking performance if you mix them. If your brakes require DOT 5.1, and you need brake fluid, use DOT 4 until you can get the fluid changed at your local bike shop. Also, DOT fluids are 'hygroscopic', meaning that they attract and absorb moisture from the environment over time. This limits the shelf life of those fluids. Only use fluid from sealed containers and dispose of older fluid that has been opened for several months. DOT fluid is also bad for paint, and you should avoid getting it on your skin or expose pets to spills. Dispose of fluid responsibly.
Mineral Oil Fluid
If your brakes require mineral oil, it is highly advisable to take your bike to your local bike shop. Unlike for automotive brake fluid, there are no 'standards' for bicycle mineral oil brake fluid. One benefit to mineral oil is that it is hydrophobic, meaning that it does not absorb moisture.
What Type Do I Have
Type of Fluid
Manufacturer DOT Mineral Oil
Avid X
Bengal X
Clarks X
Formula X
Giant X X
Hayes X X
Hope X
Quad X

When in doubt, take your bike to a local bike shop and let the pros do it right.