Disc brake loses pressure

Hey smart people, is it normal for a brake to gradually lose compression on a long descent? If I am maintaining pressure on the lever, it slowly moves toward bottoming out, over about a 20-second interval. I have to fully release the brake for it to reset to the point of full power, which is awkward - to say the least - on a steep downhill. I thought it might need a fluid refill, but that did not correct the issue. I know that the force on a brake system is different on a unicycle than a 2-wheeler because we pedal while applying the brake, but I don’t know how/if that might contribute to my issue. Any thoughts?

For sure not normal. Hydraulic brakes should not loose pressure during normal (ab)use. Disc/pads may heat up to a point where they loose power (called fading) but that should not cause a loss of pressure. Theoretically the brake fluid will start boiling at some point and this will likely lead to a loss of pressure and possibly damage to the seals in the brake system - but I’m not sure this can even happen in real life scenarios.

Braking continuously for just 20 secs should obviously not cause any issues. If you did a proper bleeding of the brakes eliminating any air in the system and your are still experiencing loss of pressure you very likely have a defect somewhere in the brake system.


Ah. I kind of expected that was going to be the answer. So my next question is: what brake do people like? Is there a favorite among muni riders? The one I have is a Shimano that I chose somewhat randomly - was my first ever disc brake. I changed over to 203mm rotors at some point as well, if that matters.

I would avoid the shimano models with servo wave action as I have heard both MTB and muni riders complain. It might be that the point of bite is less predictable.
The basic shimano brake that came with my nimbus is working just fine and predictable.

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Personally I like shimano brakes and use them on all my unis.

I use saints m820, slx m7120 and xt 8120.


@Daviddaniel, I have had this issue myself, and it turned out (or at least, I strongly suspect) that the problem was a defect seal around the piston inside the the brake lever. Replacing the brake lever fixed the issue.

I see no reason to get very expensive disc brakes for a unicycle – the forces they experience are mostly much smaller than on a bike. Mid-price range (Shimano Deore or similar quality) components should do fine, just stay away from the cheapest models.


Sounds like a defective piston seal. Over long periods of braking it is normal that it loses a bit of pressure (it’s a dynamic seal, some leakage is unavoidable), but not over just 20s.

For my hope e4, I just did a lever rebuild with new seals, Shimano unfortunately refuses to sell those as spares and you have to change the whole lever assembly…

I know many that quite like the servo wave levers, having less dead lever travel is nice. I’ve found that shimanos require a level bleed a bit more often, but then they stay relatively consistent.

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Well, then it is just a matter of preference. :grinning:

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Thanks much for your insights, guys! I guess I will be doing a lever replacement. Always gratifying to learn a new maintenance procedure :wrench: I’ll follow up with the results!