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I've had my bike (GT Avalanche 1) for maybe four or five years now. I've changed the pads once, removed and cleaned all parts on the bike a few times now, but I've never touched the brake fluid. Does this need to be changed, and if so, why and how?

I couldn't honestly say whether performance has deteriorated over time - they still stop the bike and I don't notice any fade - I'm just curious.

Brakes are Tektro Auriga Comp.

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  • It's recommended to replace auto brake fluid about every 5 years, so it would make sense to replace the fluid in bikes at least that often, if it's used regularly.
    – Daniel R Hicks
    May 24, 2012 at 11:51
  • A good question. I asked about it when shopping for a bike in LBS, and they said it would not need replacement. No reasoning, so I'm not sure if it was a valid answer.
    – Imre
    May 24, 2012 at 15:19
  • I will say kudos on keeping a set of brakes that long. I think i change parts and bikes every other year or so, and tend to avoid maintenance by upgrading and selling the old stuff. Not intentional, but thats what happens.
    – Matt Adams
    May 25, 2012 at 16:46
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    Well my bike has been pretty good, but it's definitely headed the way of Trig's broom from Only Fools and Horses, if you get the reference - (20 heads and 10 handles!)
    – Mick Sear
    May 25, 2012 at 17:45
  • IMO, zenbike's answer is incorrect with regard to mineral oil. DOT fluid (as found in Hayes and Avid brakes) is hygroscopic which means they absorb moisture from the atmosphere under normal humidity levels. Mineral oil (as found in Shimano, Tektro and Magura) doesn't. More info here: forums.mtbr.com/brake-time/dot-fluid-vs-mineral-oil-432610.html
    – Rob King
    Jul 20, 2012 at 13:42

4 Answers 4

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Mineral oil based Hydraulic fluid will build up water condensation in the hoses over time. How much time depends on a lot of factors, like humidity, temperature, bike storage conditions, and similar things.

After 5 years, I would expect that the level of contamination would be high enough to be noticeable, and that the fluid should be replaced. The specific instructions for that brake are found here.

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  • Awesome - marking this as answered thanks to the link.
    – Mick Sear
    May 25, 2012 at 6:45
  • This answer is totally wrong, cf. this article
    – Standaa - Remember Monica
    Jan 11, 2017 at 18:44
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    @Stanislasdrg You are partially correct. Mineral oil is hydrophobic, which means the fluid itself does not absorb water. However, water condensation can still build up in the hose without absorbing into the fluid like it will with DOT fluid. That said, this answer is probably misleading since contamination of the fluid could be read as the fluid absorbing the water, rather than "external" contamination.
    – zenbike
    Jan 12, 2017 at 14:21
  • @zenbike That's what I guessed you meant. But your answer doesn't explicitly states that a cause for this would be a leak, let it be in the cable or rings somewhere. There is a lot of confusion between oil types, I thought your answer deserved an update :-)
    – Standaa - Remember Monica
    Jan 12, 2017 at 14:47
  • It doesn't state that the cause could be a leak, because this type of contamination is still a condensation issue. I also didn't state that the water contamination was absorbed into the fluid.
    – zenbike
    Jan 12, 2017 at 14:50
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Late posting as the accepted answer (zenbike's) is just wrong.

Unlike DOT fluid, Mineral Oil is hydrophobic and does not absorb moisture from the environment.

Taken from this article , which contains a lot of information on brakes' oils.

Your brakes being Tektro Auriga, they use Mineral Oil. As a result, it's totally possible that your brakes keep on working properly after five years. Especially if your bike has been looked after. If the lever travel didn't change notably and you still feel the same response, there is probably no need to bleed them.

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    Coming back to this years later, I still own the same bike, and I still have the same oil in the pipes. I've never touched the brakes other than to replace the pads twice. Still going strong after 10 years or so. I recently cycled the Ridgeway path and the only casualty was the bottom bracket bearings (again).
    – Mick Sear
    Oct 23, 2017 at 13:07
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If your lines dont leak, you should be set.

Contamination of brake fluid (bikes and cars actually) is caused by exposure to air and dirt. Sealed lines should be good for 5 years easy,if not longer. I would only replace if the lines got a crack or cut in them.

Because bike brakes have a very solid seal, and no real massive reservoir, they are less prone to contamination then a car. Since cars have a large reservoir usually under a simple pop top or screw cap with a light seal, they see a bit more dirt and air in the fluid.

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The main reason for replacing the fluid is not because brake performance may have become worse, but to prevent corrosion of brake components due to water in the fluid. This problem is more severe with DOT type fluid, but can occur with either, especially if the bike is used/stored in wet conditions.

Replacing the fluid about every 5 years would seem to be reasonable -- not a terrible burden, and not being over-zealous.

Keep in mind that the size of the reservoir is proportional to the size of the overall brake system, and really has little bearing on how rapidly contamination may occur.

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