Spongy gravel bike disc brakes

I have Shimano disc brakes on my gravel bike. I bled them a few months back and have done about 500 miles of road riding on them. I did Mammoth Tuff over the weekend and had some issues. During a 17 mile shakeout ride, I rode over some washboardy gravel roads and my rear brake went super spongy to the point where full travel didn’t get my any braking. I took it to a local bike shop (shout out to Footloose Sports in Mammoth for helping me out last minute) and they did a brake bleed for me and the brakes felt great! During the race the next day, I noticed the rear brake went super spongy after 20 miles or so.

After the first time, I just assumed I did a poor job bleeding the brakes and something about the bumpy roads introduced air to the lines or something. I trust the bike shop folks to have done a good bleed so now I am just confused.

One other piece of info, I found that if I pump the brake lever a bunch I can get it to feel normal again.

Any ideas? I am hesitant to go to a bike shop without a better idea of the issue since I worry they will just bleed the brakes again and I might run into the issues next time things get bumpy.

Thanks for the help!

I take it you have checked out the pads? I have only had hydraulics a year now and when they started getting spongy I changed the pads and fortunately no bleeding was needed.

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Might be a leak in the system somewhere that is allowing some air in/fluid out. Even one tiny bubble will ruin brake performance. Unfortunately the likely culprit for this are the seals inside the lever or caliper which are not cheap or easy to fix, but it could also be either the hose or fittings.


Is there any oil lightly leaking from the lever? Check u der the brake hood. There is a known problem with Ultegra / GRX levers where the seals in the lever body can get blown if you push too forcefully against the brake pads if you are trying to open them up.

I had this happen to me twice last year.

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As a hack in the meantime, store you’re bike with the brake levers fully squeezed and put something to hold them that way until you’re next ride. This will make them feel great again, but it’s a temporary fix.

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@HLaB I will double check them. They were moderately worn when I bled the brakes myself a few months ago but seemed fine enough so I left them as well. I didn’t think it was them since I couldn’t think of a reason why worn brake pads would lead to spongy brakes. Were your pads super worn when you changed them or do you have any ideas why they were causing spongy brakes?

@MI-XC Thanks! I heard that was a fix but didn’t know it was only temporary :frowning:

@Power13 hmmmm, that might be the issue. I’ve had issues with the pads rubbing due to what I thought was not fully retracting and I might have pushed the pistons too hard when I removed the wheel and pads to reset the pistons. I assume the only fix is to replace the lever? Will this only reproduce by braking hard on rough terrain or should I see fluid under the brake hood if I do an extended brake apply, similar to what MI-XC suggested?

@JoeNation Do you have any ideas for how to diagnose if it is a leak and where the leak is? I imagine an extended hold should result in fluid coming out the leak and I should just be able to see it, right?

If that is the issue, you can replace the seal but you’ll probably need a shop to do it. Shimano can send out replacement seals. They know what the problem is.

Here is short video showing the issue. Check under the brake hood and see if there is any oil there.

The only solution is to replace the seal or the lever (I did both and almost immediately blew the seals on the new lever….causing me to blow my own gasket. :woozy_face:). It feels spongy because you are losing fluid. Holding the lever so the pads are against the rotor will only hasten the process as it will cause more fluid to leak.

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It may just be that they need bleeding again. Not inconcievable that a little air could have crept in when the shop did it, and that would account for the sponginess and need to pump it a few times to feel normal.

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Spongy isn’t usually the pads. It’s the bleed or a leaky.
If someone tried to push the caliper back in with the top port closed, it really easy to blow the shifter-side seal. You open the top port then put the bleed block in.
Shimano road calipers have been know to leak on the piston side.

Otherwise, the bleed is the issue. Like was said store the bike with the levers pulled with a rubber band or zip tie. A few times, jiggle the lines, remove the band, flick the levers, repeat. bleed. The bleed should be from the bottom up. Make sure there’s no air in the syringe or line. At the end of the bleed, squeeze the lever, push with the lower syringe and hold, and release the brake, repeat.

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Thanks @Power13. I will see if I am leaking fluid like shown in the video.

@jfranci3, when you say

You open the top port then put the bleed block in.

You are talking about the right way to push the pistons back, not a way to fix a leaking seal in the shift lever, right? I assume I need a new lever or a new seal to fix that?

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The would be a cause of a leaky master cyl (shift) which would than cause the spongy feeling. I didn’t look at the vid, sorry. But yeah… that vid a few lines up is a blown MC it looks like. Any hydro brake can do this btw.

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Could be sticky piston(s) that won’t let the pads fully engage, pumping it would maybe push it out enough to work for a time.

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happening two times is a bit of a red flag for me. I’d think leak.

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