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What now? Absolutely stumped by XT M8000 brakes

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What now? Absolutely stumped by XT M8000 brakes

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flag EasterA
Posted: Sep 3, 2019 at 13:33 Quote
Let me start by saying that I've spent the past two weeks scouring the internet for answers to my problem. I've tried everything I came across and have seen no improvement to my issue. So now I'm finally posting to see if anything new comes up.

The issue:
Shimano XT M8000 rear brake with extremely low braking power, very spongey feel in the lever, sluggish lever return.

Fixes attempted:
1) complete bleed (by the book), 5 times in total at very points while troubleshooting this issue = No improvement to stopping power or lever feel.
2) thorough cleaning of rotors (ice tech), tried metal and organic pads = no improvement to stopping power or lever feel.
3) complete lever tear down, thorough clean and rebuild = slight improvement to lever return.
4) complete caliper tear down, checked pistons, checked seals. There were signs of mineral oil seeping past one of the pistons. Seals, however, appeared to be in good shape. Cleaned everything up, very lightly lubed the seals and pistons (along the circumference), reassembled = does not appear that oil is seeping past the piston anymore but no performance improvement

Current status:
Rear brake still has very little stopping power. In the lever, resistance becomes moderate about halfway through the stroke, but you can still pull the lever all the way to the grips with one finger. It never has a solid, firm feeling. I did notice during one bleed that there is some flex in the caliper when actuating the brakes. This could account for some of what I'm feeling in the lever, but as I said, there's still very little stopping power. All of the components look to be in working order, nothing obviously out of spec. Just put brand new pads in and have not yet bedded them. I will update when that is complete if there is any change.

Anybody know of something else I can try? I've ordered new brakes but my plan was to get these running and put them on another bike
flag Reaperactual
Posted: May 3, 2020 at 5:07 Quote
Sounds like you tried everything. Long shot but maybe hose has a very small leak somewhere, not unheard of for this to happen.
flag ajlutz210
Posted: May 3, 2020 at 5:58 Quote
Yeah, you may have a leak where the hose connects on either end. It's more probable that the leak would be on the lever end if you crashed or knocked the bars. If you have enough extra length on your hose, you can cut off the end and use a new ferrule/nipple thingy to reconnect the hose. If you don't have enough extra length, you would need to cut a new hose.
flag Reaperactual
Posted: May 3, 2020 at 6:25 Quote
Had the tiniest nick in a hose once. Think it could have been caused by a blade when cutting a cable tie off, (same location as cable boss). Took months to realise the problem.
flag Impatient
O+
Posted: May 4, 2020 at 15:09 Quote
I’m sure you thought of this, but any chance your fluid is bogus, contaminated, or wrong spec? Have you used this same fluid in another brake without problem? The other thing you “could” try is “going moto” swapping the levers to the opposite wheel, and see if the problem stays with the original lever, or original caliper...but then there is some risk you’ll end up with two bad brakes
flag JonDud
Posted: Jul 25, 2020 at 6:34 Quote
EasterA wrote:
Let me start by saying that I've spent the past two weeks scouring the internet for answers to my problem. I've tried everything I came across and have seen no improvement to my issue. So now I'm finally posting to see if anything new comes up.

The issue:
Shimano XT M8000 rear brake with extremely low braking power, very spongey feel in the lever, sluggish lever return.

Fixes attempted:
1) complete bleed (by the book), 5 times in total at very points while troubleshooting this issue = No improvement to stopping power or lever feel.
2) thorough cleaning of rotors (ice tech), tried metal and organic pads = no improvement to stopping power or lever feel.
3) complete lever tear down, thorough clean and rebuild = slight improvement to lever return.
4) complete caliper tear down, checked pistons, checked seals. There were signs of mineral oil seeping past one of the pistons. Seals, however, appeared to be in good shape. Cleaned everything up, very lightly lubed the seals and pistons (along the circumference), reassembled = does not appear that oil is seeping past the piston anymore but no performance improvement

Current status:
Rear brake still has very little stopping power. In the lever, resistance becomes moderate about halfway through the stroke, but you can still pull the lever all the way to the grips with one finger. It never has a solid, firm feeling. I did notice during one bleed that there is some flex in the caliper when actuating the brakes. This could account for some of what I'm feeling in the lever, but as I said, there's still very little stopping power. All of the components look to be in working order, nothing obviously out of spec. Just put brand new pads in and have not yet bedded them. I will update when that is complete if there is any change.

Anybody know of something else I can try? I've ordered new brakes but my plan was to get these running and put them on another bike

You might have cracked the little internal plastic piece that is notoriously garbage on the m8000's. It's the piston guide, and when it snaps, the brakes still "work", just not very well
flag DoctorJD
Posted: Aug 5, 2020 at 11:37 Quote
I could have written your post. I have two long threads over on mtbr.com if you want to read the novel. The spoiler ending: I replaced the brake. I've been a long-time Shimano user, and I was convinced there was something going on other than a quality issue. After talking to several people, there just seemed to be a reoccurring issue with this brake. Mine had a faint fluid leak and I didn't really pick up on it until I started looking closely at the back side of the pads. It was very subtle, but oil was there. And this, after replacing the seals and pistons.

I turned the page on the XT and bought a new LX 4-piston to replace it. I couldn't be happier. I dumped a good bit of time and money into trying to make that other brake work. It was an exercise in futility. I hope you have better luck, but be prepared for the worst.
flag Reaperactual
Posted: Aug 5, 2020 at 11:53 Quote
Had the odd issues with my SLX M7000 rear brake. No leaks but wandering bite point, not really an issue just irritating. Reluctant to replace caliper in case issue lies elsewhere but am considering it.

Been posting for answers myself with no real solution or definitive answers as to why. I've probably read your post at some point but could you post a link to mtbr thread please?
flag DoctorJD
Posted: Aug 7, 2020 at 6:34 Quote
Reaperactual wrote:
Had the odd issues with my SLX M7000 rear brake. No leaks but wandering bite point, not really an issue just irritating. Reluctant to replace caliper in case issue lies elsewhere but am considering it.

Been posting for answers myself with no real solution or definitive answers as to why. I've probably read your post at some point but could you post a link to mtbr thread please?

Here's my thread from mtbr.com it has a link to the original thread. Note that this went on for about a year. I went from being fully convinced that I didn't have a fluid leak, to coming to terms with the fact that I did. I went through multiple sets of pads, and endless rotor resurfacings. So much time and money wasted.

https://forums.mtbr.com/brake-time/possible-rotor-become-contaminated-beyond-repair-1144943.html
flag Reaperactual
Posted: Aug 7, 2020 at 7:15 Quote
Thank you DoctorJD. I will check it out, not really my symptoms but it may help. Wink
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