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I know that the two main categories are the mineral oil (Shimano, Magura) and the DOT type (Avid, Hayes).

But what is the difference between the DOT 3, DOT4, DOT 5.1? Which one should I choose for average mountain biking?

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2 Answers 2

up vote 4 down vote accepted

From Wikipedia:

Part of the difference is boiling points:

               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     270 °C (518 °F)     190 °C (374 °F)

3, 4, and 5.1 are glycol-based, and must be changed periodically, as it absorbs water. 5 is silicone-based and does not need to be replaced.

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Be cautious. That's just the minimum to make the rating. Some of the higher boiling point fluids sound "better" due to that one factor. Realize however, that many of those absorb moisture much quicker and will be down at wet boiling point while some other fluid will be still sitting up closer to dry... Learned this the hard way on the racetrack... The big dollar stuff was water saturated (and would boil) after a week in the car, whereas the cheap factory stuff stayed dry and would not boil even after a year in car... –  Brian Knoblauch Oct 20 '10 at 20:13
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To answer the "Which one should I choose for average mountain biking?" part,

You don't really "choose" your brake oil, it's specified by the manufacturer. The oil is chosen depending on the heat output of the brake and the general construction of the system. A different oil than what the manufacturer recommends might affect/break the cables, seals or some other parts of the system. Check your brake's manual (most likely available online if you lost it) to know what you need.

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Its manual says "Use only DOT 4 or DOT 5.1 fluids". Based on the other answer I will use 5.1. –  csadam Oct 17 '10 at 10:46
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