XTR M9000 rear brake refuses to bed in - behave

[open roader]

I like to perform my own bicycle mechanical maintenance, both to save money and because I really enjoy it. I have been doing my own mechanics on both my road and MTB for a decade so I have some experience - most of it very good..........

However, back in early Oct '18 I commenced a Yeti SB6c frame build. I purchased a pair of new XTR 9000 series brakes. I also purchased a pair of Shimano Ice Tech rotors for the build.

My problem with the rear brake is now truly epic. I've put together 3 other hydraulic brakes MTB's before this build - no problems with anything before, a bit of brake squeal to begin with but after some braking runs all runs as it should. Occasionally I've had to sand back a pair of pads but nothing like the current issues I'm having.

I've put in so much work and ridden up my local steep hill so many times now that I'm just jacked off with the entire thing.

The initial build was with Ice Tech rotors - front brake no problem - rear brake shocking squeal - mega decibels like none I've heard before - really loud stuff.

So I have done the following over the past 3 months :-

Emptied the rear brake line and replaced with brand new mineral oil - bled it out for 45 min then left it over night to bleed out the last bubbles.
Cleaned the entire caliper and rotor with brake cleaner
Replaced the OEM resin pads with brand new Shimano resin pads
Performed multiple bedding in runs on the new resin pads - no difference
Replaced the resin pads with Shimano metal pads
Performed even more bedding runs - no difference
Replaced the caliper mounting bolts
Bled the entire line out overnight again
Ditched the Ice Tech rotors and replaced with standard solid steel Shimano rotors
Re performed endless bedding in runs with both resin and metal pads - still nothing
Re-bled the line out again overnight
Cleaned the piston to pad interface with brake cleaner
Re- tensioned the rotor mount bolts
More bedding in runs - endless bedding in runs maybe 100+ brake actuations over 2 weeks down a steep hill ........... then ride back up to the top for another seemingly idiotic attempt
Gave up altogether and took it to my local bike shop - he had it for 2 weeks - I picked it up last Friday - no fix, no ideas and luckily no charge.

The rear brake actually works quite well, has stopping power and good lever feeling but the noise is absolutely horrendous to the point where I've received multiple complaints from local residents about my bedding in runs down the local hill. I just can't keep on going bedding in runs there has got to be something more to it than simply pad/rotor interface issues.

Am I right to be seriously considering ditching the rear caliper in favour of a new one? I'm thinking I must have a dud caliper as I can't think of anything else to eliminate.

I HATE being beaten by this sort of thing. I've invested dozens of hours into this one issue and have made a pariah of myself to some of the local residents. The Yeti is something I've always wanted but it's just so disappointing to have it virtually unrideable still after all this time...........

[boots2000]

Did you ever go to new pads and rotor at the same go?
I had a similar problem with a brand new 9170 caliper with a new rotor.
After lots of sanding and re-tries changing the parts separately, I went with a new pad set and a rotor from an older wheel.
Bedded and worked perfectly-
Rotor from new setup worked on anther bike perfectly.

[chris7ed]

Try front gear on the rear. Maybe isolate that way with pieces that you know work ok.

[Applesauce]

What colour are the caliper pistons? Shimano experimented with phenolic pistons in that brake, most of which failed by seizing; the brake would still work, but the caliper would drift unpredictability depending on how which or either or both pistons were moving. They were pretty cagey about it, but replaced the brakes as needed with one with their traditional ceramic pistons. But I’ve seen plenty of issues with those too: they’re prone to cracking, which will leak if it doesn’t fail outright. XTR carries a three-year warranty, and Shimano often deals with mystery issues with outright replacement (which unfortunately for you means rerouting the brake line through Yeti’s super-shitty ports).

Many Shimano rotors and pads are prone to oxidation (I think, at least, and Shimano ain’t saying), but it’s usually a slower process than what you’re describing. (What I mistake for oxidation may also be piston square seals that leak just enough to contaminate a pad/rotor combination...but again, Shimano ain’t saying.)

It’s possible the post mount faces on your frame aren’t straight, but in modern brakes this will usually result in noise for a bit, and then just uneven pad wear.

I can almost guarantee you, you don’t need to bleed the brake again. A bleed will never cause pad issues (unless you spill fluid all over the pads in the process).

What you’re experiencing is normal for the intrepid home mechanic: you just don’t see a big enough sample of anything to recognize trends in individual products or systems. Try a high quality shop, but don’t necessarily consider it admitting defeat, as you might learn something that will make your next home project go much smoother. Tread cautiously however: most shops these days aren’t automatically friendly to a whole whack of internet-sourced Shimano products. But any shop worth it’s salt should be able to get past it, and you should be prepared to pay to have your issue fixed properly.

[open roader]

Finally I've had a home grown victory!

After all these months the tell tale sign was in front of me and the fix was sitting in the bottom of my spare parts box!

I calmed down and looked at it again on Christmas Eve as it was too hot to do anything else. I noticed the front rotor brake track mark was perfectly aligned on the rotor brake rim but the rear brake track mark was actually just a tad closer to the centre of the disc. The rear caliper was sitting down lower on the mount than the front (que Applesauce try the functioning front equip on the rear - except in this case I've avoided dealing again with the diabolically bad cable routing on my Yeti :-)

I added a 0.6mm steel washer between the caliper and the frame mount and it fixed the noise in an instant. I can't believe the difference between banshee scream and silent running was sitting right under my nose all the time. I've tried an emergency big breaking effort run and I have zero noise - just a perfectly functioning brake!

Now I have a new bike to rise just in time for Christmas!

Yeti-SB6c-Dec-2018-email.jpg

[Drew]

Hot damn. Crack a beer.