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I have a few bikes with shimano disc brakes. I know changing the fluid is something that can be done and I could probably work out how to do it but I don't know what to look out for to know its time to change the fluid. I know someone who took in their 8 month old mountain bike to the store for a service and they said the fluid needs changing. Is this too soon? Are there any changes I can notice in my brakes that will let me know its time to change the fluid?

  • Possible duplicate of Do I need to replace hydraulic fluid in my brakes? – mattnz Feb 25 at 19:58
  • 8 month old bike - if it had brake problems, hope they did not pay for the fix as it would be covered by warranty. If it had no brake problems, they got ripped off - find another bike shop. – mattnz Feb 25 at 20:02
  • @mattnz agree this question has been asked before, but your proposed dupe does not have an answer anywhere near as good as khendrickx's. – Argenti Apparatus Feb 25 at 22:05
  • @ArgentiApparatus So do you think it would be a good idea to close the other question as a duplicate of this one? – David Richerby Feb 25 at 23:54
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    I don't think they are duplicates. One is asking if they do need to be changed and this one is how often – Qwertie Feb 26 at 0:47
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The suggested interval to bleed your brakes seems to different among manufacturers. While SRAMs techdocs explicitly mention to replace the fluid every year, Shimanos techdocs (p20) does only recommends to replace the oil if it becomes severely discolored. For recreational use, I think you're safe bleeding once every two years but depending of the usage and storage it might be quicker.

Magura on the other hand claims their Royal Blood oil (p21) does not age. I've ridden an MTB having HS33 brakes with 10 year old oil and indeed had a firm braking experience.

Are there any changes I can notice in my brakes that will let me know its time to change the fluid? Replacing the fluid before the regular maintenance can be required if air got in the system. Magura suggests to replacing the oil if:

  • the brake does not respons immediatly when the brake lever is actuated
  • pressure point is not clearly defined, it is spongy or does not remain constant
  • the brake hose has been changed

If you have multiple bikes and like to do some own maintenance, it's even worth looking into buying your own bleeding kit. Shimano has nice technical documentation and there are various videos online giving you a feeling how to bleed your specific brake type.

Small sidenote, regarding storing your bike: ideal you put the flat and upright, so there is less chance for air coming in the brake system. While transporting the bike be sure not to press the brake levers without having the wheels in your bike, as it can cause the pistons coming out too far.

  • To my knowledge the reason for these differences is whether the fluid is mineral oil based (shimano) or DOT4/DOT5.1 (sram). The latter are glycol-ether based wikipedia. The glycol based fluids absorb water and one wants to keep the brake system free of water (brake hot → water boils → gas in the system → brake power gone). Thus DOT4/5.1 based systems are expected to require service more often than mineral oil systems . – pseyfert Feb 25 at 18:34
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    @pseyfert DOT 5.1 brake fluid deliberately absorbs any water that gets in the systems to prevent it from forming vapor when the caliper gets hot. – Argenti Apparatus Feb 25 at 22:20
  • @ArgentiApparatus thanks for the correction, does that then have anything to do with the inspection interval? (expected to be saturated after a year?) – pseyfert Feb 25 at 23:01
  • @pseyfert the only advice I’m willing to give wrt hydraulic brakes is do what the manufacturer recommends. I’m sure SRAM worked out bleed interval that will keep the vast majority of riders riding in average to wet conditions safe. If you live in Arizona or the McMurdo Dry Valleys then you probably have a massive margin of safety. – Argenti Apparatus Feb 25 at 23:32
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It is common in the automotive industry to change the fluid when problems arises. They have the longest experience in the field and that's what I do. Change the oil when the brake lever feels spongy or less firm.

One trick to have the firmest lever possible is to block the lever with a cable tie overnight, making sure that it is in the highest position, so that all the air can go easily toward it(bike in oblique or vertical position, it depends on your routing).

In the mtb world some people will change fluid every 6 months to 1 year, this is a good practice if you race.

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    It's not clear that car brakes are a good comparison: they have to stop ten times the mass from four times the speed, with a lot of potential third-party liability if things don't work properly. – David Richerby Feb 25 at 23:58

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