Firstly apologies, I know there are umpteen threads on the issues with M785 and M8000 XT brakes but despite lots of research I cant seem to get to the bottom of this and not being very technically minded it would be good to get some input.
I have a stumpjumper fsr 29er which came with m785 XT brakes. From new the rear brake would rub and squeal on the rotor after I had released the brake lever, this was particularly prevalent when the brakes were warm, e.g. during longer descents. Apart from the irritating noise the brake worked well though.
Having ruled out a bent rotor, contamination and a few other options I had the pistons inspected but nothing seemed up. I eventually got fed up of the noise and took the bike back, where the brake was replaced under warranty with an M8000 XT version.
I hadn't ridden the bike much over the winter, and now the new XT brake does exactly the same thing under the same circumstances and I can even replicate it by dragging the brake to warm it up and then letting go of the lever. This new brake (unlike the old one) also suffered slightly from pump and fade in the lever.
So far I've tried the following:
- I swapped the rotor over from another bike, made no difference.
- Explained the issue to my LBS who have checked and adjusted the caliper position, which made no difference
- Checked spoke tension, pivot bolt tension, thru axle tension, made no difference
- Just had the brake bled which has cured the fade/pump issues and the performance of the brake is great, however it is still making this squeal although not quite as bad.
To my untrained eye (ear) a slow returning piston would seem like a perfect answer to why it was happening after braking but given all of the above I cant understand why or what else to try and I am sceptical as to whether I've been unlucky with consecutive versions of 785 and 8000 XT brakes.
I have M785 XT brakes on another bike which have been faultless for over 3 years.
Can anyone offer any advice or options to try, other than buying a different brake?
Thanks in advance
I'd be tempted to swap the known good M785 rear onto the offending bike and see if the problem persists as you've had the same plus a possible seal issue on two sets now. Just to be 100% sure the bike itself isn't hiding a problem before you start...
could just be a symptom of a failing caliper
or a simple thing could be air in the caliper expanding with the heat and pushing the piston out.
Take them off the bike and give them a really good clean, carefully push the pistons out, placing something in between to stop them fully popping out and give them a good clean with a cotton bud and lube the seals with some silicone oil if you have some and then carefully push them back in. Do a full bleed with fresh shimano oil and then try again.
Thanks andyl and vince....
I'm really reluctant to dismantle the other brake, but agree it could be a good way of checking whether it is the brake or anything else.
They've just had a full clean, piston lube and bleed. The issue is still there, although it doesn't seem quite as bad as before , so possibly there is still a tiny bit of air in the system?
I agree with andyl above - except, just use mineral oil to lube the piston. I've had similar concerns with an XT8000 brake - the front is A ok but the rear always seems to have a piston sticking - not the same one. Generally occurs after longer descents, no squealing but the rotor rubs after the descent. It bugged me after the first few months so I bought a new caliper but it was just the same. Brake lever also varies in 'feel', sort of pumps out on descents, no loss in braking force. Possibly the widely publicised issue experienced with these brakes. Bleeding improves it a bit.
Any road - alternately pushing one piston out, lube, push back and then the other, push out and clean...usually runs ok for a month or two.
I also think the rear rotor seems to warp more with heat - RT76 type, maybe I got one a bit out of tolerance. The front is ok and on my other 2 bikes I've got the RT-66 rotors which I think work better. Weigh the same too !
I've had similar concerns with an XT8000 brake - the front is A ok but the rear always seems to have a piston sticking. Generally occurs after longer descents, no squealing but the rotor rubs after the descent. Possibly the widely publicised issue experienced with these brakes. Bleeding improves it a bit.I also think the rear rotor seems to warp more with heat - RT76 type, maybe I got one a bit out of tolerance. The front is ok and on my other 2 bikes I've got the RT-66 rotors which I think work better. Weigh the same too !
steel4real - ^^^This is exactly what I'm experiencing. In a way its good to know its likely to be this and I can manage it with the occasional bleed but the noise does drive me crazy when it happens!
I have Ice-tech (RT86) rotors on the stumpjumper and the standard (RT76) on my other bike. I swapped them over last week to try it and it was just the same.
Quick question:
Do the SLX M7000 brakes suffer any of these issues?
I'd suspect a sticking piston in the master cylinder, if it's slow to return then the caliper piston will be too, and if it doesn't fully return, then it's seal will block the fluid's return to the reservoir, causing issues such as pump-up when the fluid heats and expands, and long lever travel as the pads wear with no extra fluid to compensate.