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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 '12 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 '12 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 '12 at 16:46
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 '12 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: – Rob King Jul 20 '12 at 13:42
up vote 6 down vote accepted

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 '12 at 6:45

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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