I see lots of questions about hydraulic brake-specific maintenance. I.e. related to the hydraulic system itself, not the pad/rotor interface. (For this question let's ignore the latter since that's generally applicable to mech disk brakes as well)

Most of these questions are spurred by some specific issue: mushy levers, leaking oil, unsuccessful bleeds, etc.

That got me wondering, assuming your hydraulic brakes seem to be working perfectly, should you ever do any kind of preventative or regular maintenance on the hydraulic system? E.g. should a brake bleed or replacing seals or anything else be done every X number of km/seasons/years even when everything is working fine?

As a specific example, I've got a fat bike that's approaching 7 years old. Being a fat bike, it's still low mileage - around 700km. It's got SRAM Level hydraulic brakes. I've never had any issues with the hydraulic aspect of the system: levers are still firm and apply lots of force to the pads with good modulation and no mushiness. Should I leave well enough alone until that changes? Or should I do some preventative maintenance now?

I guess the motivation for this question is wondering if the hydraulic system can suddenly fail catastrophically with no warning if not maintained. Or do all common failure modes telegraph failure somehow, e.g. with slowly decreasing brake effectiveness?


  • I'd call cleaning/lubricating pistons preventive maintenance on the hydraulic system, but would you? It's not something I've had to do yet, but maybe I should, and a friend needs to in the next day or 2.
    – Chris H
    Commented Jun 22, 2023 at 9:57

2 Answers 2


In the case of SRAM, the brake liquid is DOT 5.1, as in automotive brakes (and not mineral oil like Shimano or Magura). DOT 5.1 is hydrophylic and absorbs the ambient water vapor from the air. That impacts the fluid performance as the boiling point gets lower (from 290°C to 190°C) as the moisture content increase (DOT fluid has a dry boiling point and a wet boiling point). Mineral oil by contrast is hydrophobic, and if water gets into the system (which doesn't occur "spontaneously"), it will remain separated from the fluid, and the boiling point for the system becomes water's one (100°C). If the fluid boils, the liquid turns into vapor, that is a compressible fluid. The brakes will then become "mushy", since you'll have to first compress the gas, before actuating the pistons.

The risk linked to the fluid (there can be other as well, linked to seals) is likely to occur if the brakes get very hot (in other words, when you are using them) — and they will operate normally if they remain cool. DOT 5.1 brakes will have "spontaneous" degrading performance over time. Mineral oil brakes are less prone fading if not used, but in case of failure, they will become mushy sooner when used.

Most fat bikes are ridden in "high rolling resistance conditions" (from the tires or the surface), and not as fast as other bikes, so the brakes are less likely to reach these temperatures, but that's a possible failure mode.

  • Interesting, I didn't know that. I was hoping answers to this question would be widely applicable to hydraulic brakes in general, so this is good info even if it's less likely for fat bikes specifically. Would you expect that could cause a sudden or moderately rapid failure (i.e. within one ride or descent)?
    – SSilk
    Commented Jun 22, 2023 at 7:46
  • Also, given this hydrophilic quality, is the expectation that brakes using DOT 5.1, even in good condition, are constantly but very slowly absorbing water? E.g. does this quality make them pull water in around the seals? Or would water ingress only be expected to happen when something else is wrong, e.g. a seal has degraded?
    – SSilk
    Commented Jun 22, 2023 at 7:48
  • I was about to edit the answer to add a comment about mineral oil (the 100°C I mentioned applies to mineral oil in fact), but yes, it is normal for DOT5.1 to absorb water, it's not a symptom of defect joints.
    – Rеnаud
    Commented Jun 22, 2023 at 7:52

Yes, there's something you should do. You should use both brakes occasionally.

If you fail to use a certain brake, it's possible the piston will start having issues such as not retracting properly, creating a brake that drags and steals few watts of your power. This may be fixed by moving the piston many times to both directions, holding the other piston in, and lubricating and cleaning the "lazy" piston seal with brake fluid and a cotton swab.

This is more likely to happen on a rear brake of a road bike that's ridden only on paved roads where 100% of the braking power comes from the front brake and 0% of the braking power comes from the rear brake in ideal conditions. On mountain bikes, presumably the rider uses them on loose terrain, meaning you absolutely need to have both brakes, or else you risk crashing due to a locked front wheel.

So, if you're a road biker, remember to occasionally use the rear brake too. This means it's useful to ride on steep unpaved downhill roads, because it keeps not only your brakes in good condition, but your skills also in good condition.

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