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Firstly I'd like to mention that I don't think this issue is related to contamination (at least from sprays etc) and I'm aware of the following 2 threads:

Disc brake stop working after wash,

How do I get my hydraulic disc brakes working well again

I'm having trouble with the rear brake on my mountain bike. It's running Shimano XT (BR-M8000). The braking power seems very feeble and I'm getting a squeal as the bike slows to near-stop.

So far I have tried cleaning throughly with alcohol, brake cleaner and even taken the rotors of and cleaned them with soapy water followed by isopropyl alcohol. I've keyed the pad with emery paper and even swapped the pads out.

I've bled the brake twice and the initial feeling through the lever is good. I did my best to get all the air out and extended the reach to get a greater pull on it.

I wondered whether I'd glazed the rotors and replaced them with brand new (and cleaned) rotors as I'd been meaning to do so for some time to reduce noise.

My new rotors a some Shimano Ice Tech 160mm centre-lock so should be the right gauge. I've bedded them in without dousing with water as some do. The front brake is a larger rotor but bites really well. I can't even skid the back wheel.

I don't think the calliper is leaking but I haven't done extensive checks to see. I did get what felt like a much better brake when I got them covered in sandy mud. After I cleaned the bike with muc-off the brake was back to being pants again.

My next steps were going to be to check the brake hose for weak spots and/or bulges. Replace the hose if needed.

Last option is to empty the system and clean the calliper and remove, clean and reseal the calliper pistons.

Is there anything else I can do before I go down these routes?

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    One more though I have is a pinched or blocked hose would prevent the pressure getting to the calliper. Possible diagnostic (involving a lot of shagging around) would be to connect the rear brake line to the front lever and then the front disc and isolate the problem to Calliper, lever or hose. – mattnz Oct 12 '16 at 20:05
  • Actually that's a good shout. I'll unclip some of the zip-ties and see if I can straighten the routing out for a bit. I'll check for pinches too. I don't think the hose is blocked because when I was bleeding it the oil ran out the calliper nicely as I was getting the air out. – Chris Oct 12 '16 at 20:14
  • Is lever feel firm even when squeezing hard? Once I worked on something like this and the problem was a damaged spot on the hose where squeezing firmly would cause it to balloon out. – Nathan Knutson Oct 12 '16 at 20:43
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    Also, when you look inside at the gap, can you visually confirm that both pistons are moving and both pads are hitting the rotor, i.e. there's nothing going on where one piston is stuck and/or the rotor is getting shoved against the caliper? – Nathan Knutson Oct 12 '16 at 23:35
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    My bet is contamination of the pads and / or rotors. – OraNob Oct 14 '16 at 14:32
5

I also had noise and power issues with my m8000s when they were new. Sanding the pads and cleaning the rotors made no difference. My solution was to remove the pads, clamp them in a vise, and hit them with a propane torch for about 30 secs each. They briefly flamed up, which to me indicated contamination. Once reinstalled there was power and silence. That was over a year ago and the problem has not returned.

There is a good thread on MTBR about this issue.

http://forums.mtbr.com/brake-time/xt-xtr-brakes-get-squeal-power-loss-after-sitting-idle-6-8-weeks-902792.html

  • Wish I had a torch. I have a bazillion other tools but nothing with a flame except the gas hob in the kitchen. I think I've heard of some people cooking their pads in an old frying pan. I might see if I can jerry-rig something. – Chris Oct 15 '16 at 22:13
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    After 2 days of excellent and quiet braking I can only assume that the pads had some sort of contamination. I've been meticulous in my preparations when using any sprays (wheels off and pads out). At this point I have to assume that road grime is the culprit. I've accepted this as an answer as the suggestion is sound and the forum thread is full of people with the same bizarre issue. – Chris Oct 20 '16 at 5:31
  • I let the bike stand for 5 days and on returning to it the back brake is now next to useless. I'm going to have to diagnose this a little more thoroughly. – Chris Oct 26 '16 at 12:32
  • Any luck? I have the exact problem, front and rear, on my cyclocross. My road bike is also equipped with a Shimano ST-R785 hydraulic Di2 disc brake set and I've never experienced anything but silent excellent stopping power there... – T0TTE Aug 21 '17 at 8:03

protected by Community Nov 24 '18 at 7:17

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