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Discussion Starter · #1 ·
Hi all at mtbr, my maiden post and alas it's a complaint / problem but that's usually what I'm not about.............

I'm 50 years old, live in SE Australia and ride only easy forest trails, plantation tracks and occasional social rail trail ride weekends. I'm a cautious rider - never had more than 18" of air under me - most of the time 12" of air under me is sufficient to get me my thrills. Nearly all of my MTB riding is done in dry and partly dusty conditions - 20km/hr downhill for me is really fast..........

I built up my 2017 Yeti SB6c frameset with XTR M9000 brakes. Yeah, yeah - I know the SB6 is way more bike than I am rider but what the hell, I deserve it.

I also have a 2008 Giant Trance 0 - this one I built up with XT M8000 series brakes. The XT brakes have never given me a moment of bother - just finished the 2nd set of resin pads, a complete oil refill and bleed and the third set are now bedded in and working exactly like new resin pads work on XT - pretty damn good for a low risk rider like myself.

My problem is with the SB6............ the bike I love the ride characteristics of the most but the build that has given me the least amount of joy since it was finished in late 2017.

The SB6 :- the original set of pads (resin) I bedded in with a very gentle method, 15km/hr to nearly stand still - maybe 35- 40 repeats and not much more than 50% pressure on the lever. The front worked fine for maybe 200km of dry dusty trail riding then honked / squeaked with every application of the brake. The rear was initially quiet and worked for less than 100km before it began to do the same.

My learning curve grew upward fast. I've learned to emery paper the pads, clean, rinse and dry - sand the rotors lightly etc. Same deal - same distances before troubles started. I've used isopropyl alcohol, acetone and commerical in a can brake cleaner at different times.

I upgraded from the Avid Clean Sweeps to Sram Centerlines with new resin pads. The results got me slightly longer distance service for the front but the rear squealed from the start and never got better. The pads glazed so badly I ditched them for a new pair and this time I used a more forceful bedding in method - down a really long steep hill, yanking on the brake for a few seconds then cooling for a few seconds and repeating - 15 repeats. I got two 30km rides in before the pads glazed again but this time only making noise at the very end of the braking cycle ie walking pace to a halt.

Since then I have tried a pair of sintered pads on the Centerlines - front OK but no real initial bite - rear brake was really bad- could never get it bedded in and glazed the pads 3 times before ditching that idea.

Changed to a barely used pair of Magura rotors and upgraded size to 180mm - fresh resin pads and alas the same thing. Front works well for a couple of rides before glazing hard and the rear for about half as long before glazing hard.

My rotors are clean and well aligned. My pads have been new from the start and my sanding / cleaning attempts have been thorough. The hose line banjos are torqued right, no leaks and the pistons are well sealed - no leaks. Every time I remove glazed pads there is no rotor drag and the pistons appear to have retracted just fine.

I'm pretty much at my wit's end with this one. 5 pairs of resin brakes (Shimano and Swiss Stop) and one pair of (Shimano) sintered brakes later I'm at a complete loss. This is a 4 year old thorn in my side that I can't fix. I've attached a pic of a typical pair of glazed over pads - these are from the rear after 2 x 25km rides admittedly I hardly giving the brakes a hard workout on those two rides but you get the picture..........

I'm yet to try sintered pads on the Magura rotors and that is next but aside from that I'm thinking best to ditch the whole brake system and try something else and yet that makes no sense...... as it's the pads that are letting me down?

I like to do all my own bike maintenance, have built up 7 road bikes and 4 MTB's and I like my stuff to run as close to perfectly as possible. Howling glazed over brakes does not come even close to perfect - I've effectively had no value and certainly no joy from my M9000 brakes. I really see this as a personal challenge - even if my brake system is no good I really want to know why it's no good - it's driving me nuts.

Any suggestions - what am I missing here?

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It looks that you did everything you could and/or even more. I have no additional ideas what else you could try. In that case only suggestion left is you sell complete brake system and go with other, i.e. Magura MT5 or MT7. Sometimes its just bad luck or some illogical thing :)
 

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I'd try Shimano rotors. Odd problem, maybe take the pads out and give the inside of the caliper a good clean. Could be a small oil leak that's contaminating the pad that looks like glazing?
Every few months I'll take the pads out of my Saints and give the calipers a good clean.
I honestly think the bedding in thing is a bit over rated. Make sure the rotors are clean, put the new pads in, and go give them some hard stops. My old M675 SLXs were ferocious from the first pull.
 

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Discussion Starter · #4 · (Edited)
Thanks guys. I've had a couple of further thoughts myself.

In the past I've had a couple of really left field issues with road bike builds - things that drove me insane but I eventually stumbled upon once I threw a tantrum and pulled it all apart only to discover the oddest, most long shot type things holding back correct function and performance.

I can afford Magura MT5 or even MT7 or Hope or Sram or brand........... however, right now I'd call that admitting defeat. I really want to know what it is that's holding this back.

If the sintered pads (due to arrive in the mail sooner rather than later) fail to get some longevity then I might borrow a pair of Shimano rotors I know are sitting around doing nothing and give the 100% Shimano parts method a run.

If that fails I also figured I could switch calipers f & r and see if that makes a difference - scrutinize in daylight whilst disconnected, clean the pot faces and seal outers before re-installing and bleeding them out again - Ii have plenty of Shimano oil so why not.

"Could be a small oil leak that's contaminating the pad that looks like glazing?"

Has got to be something like this as it's becoming very apparent that no matter what I do the result is ultimately the same...............

"I honestly think the bedding in thing is a bit over rated."

I think there is some serious validity to that as my XT brakes on the Trance worked 100% fine from the day I fitted them with no bedding in (they are running on Sram Centrelines BTW)

Below :- a pic of the offending rear caliper FWIW

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The wear pattern in the photo indicates the rotor is too small relative to the brake caliper mount. Get the correct size Shimano ice tech rotor and replace the pads at the same time. It would probably be a good idea to have the bike shop do it and verify that the replacement rotor is of the correct size and that the calipers are properly adjusted.
 

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From your read you have been pretty methodical. The pads in the picture look more contaminated than glazed. I stole this pic from the web, the pad on the left is glazed, right has been sanded past the glazing. Your pic is more of a black build up, which could be grease from a hub, a very small leak in the caliper (although that sounds unlikely) or my last thought, if your riding in really dusty conditions, it could be dirt built up.
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It MIGHT be a leaky caliper. I just went through a similar experience with a SLX 4 piston.
I spent almost a year trying to diagnose a similar issue only to find that I had a leaking XT caliper. I'll tell you that the leak was so subtle that I convinced my self (earlier in the process) that there was no way I had a leaking caliper. It only takes a faint amount of oil to totally make a set of pads useless. I'm seeing more and more stories like this with Shimano brakes.
 

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I sanded the old pads and rotors, no dice. Installed new pads and rotors, the problem returned(also rendered a brand new rotor and set of pads useless) I just never thought to look on the back of the pads, but sure enough, there were faint mineral oil rings. I ended up getting a warranty replacement on the whole housing. I'm pretty sure shimano doesn't make replacement seals for calipers.
 

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As others have mentioned, the pads appear to be contaminated rather than glazed, and that wear pattern is strange. It's that gunky, grease-like build up that has me concerned, especially since it does not sound like you are going fast enough nor braking long enough to generate the heat needed to facilitate the glaze.

I don't really think this is a matter where changing out rotors will solve the problem. The centerline rotors are generally very quiet; they just tend to warp under hard braking. I have no experience with the Magura rotors, but my friends running Magura rave about them. My experience with Icetech rotors is that while they resisted warping, they were very loud.
 

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I've got several sets of Ice-Tech rotors, including one set on my (all) road bike, and despite making very heavy use of the brakes (requiring pad replacement every two to three months, and rotor replacement every year), they have been silent. I use J03A resin pads, fwiw.
 

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I agree could be a leaky piston/seal.

Once you have that sorted out, personally, I have best luck "leaving the rotors dirty" what I mean is, once they are working well, bedded in, and quiet, don't touch them. Don't wipe/clean the rotors unless you have a good reason.

Brake cleaner tends to make mine howl.

I know a lot of folks like to wipe the rotors with alcohol etc, but it's really not needed

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What you're running into is an issue my buddies have run into for years with Shimano brakes. I'm not sure if the piston material they use is slightly porous, or if the piston seals are just poor quality, but the "self-contaminating" feature of Shimano calipers is well documented amongst my group of riding buddies. Some of them love the Shimano feel so much they just warranty them, or buy new calipers hoping for a fix...but weepy pistons that slow contaminate pads and rotors are a known problem. Supposedly some of the new Shimano brakes are better, but it was always a somewhat inconsistent issue, and personally I won't risk the annoyance and cost of replacing rotors and pads and have moved on to running other brakes.
 

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My M9000s are notorious for weeping a little mineral oil at the (rough) pistons. The longer they sit around, the more they do this. You'll go ride the bike after a month have have almost no braking, or at least significantly diminished braking. It takes baking the pads in the oven to vaporize the mineral oil and cleaning the rotors with alcohol. Yep, another shimano issue. I have two bikes with these brakes, one does it more than the other, but it's definitely this issue, no doubt about it.
 
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I am having a very similar problem with my Shimano XT-8020's. It didn't take long and they were glazing and howling like yours. They were so weak I couldn't lock up the rear wheel. I tried sanding the pads and rotor and cleaning with alcohol but that didn't last. The one last thing I tried that helped the most was to take out the pads, hold them with needle nose pliers and burn them with a cigar torch lighter until the smoke stopped coming off of them. Figured it was a last ditch effort but it worked. Problem is the issue came back so now I will have to check for a leak. I think the smoke must be the mineral oil burning off.
 

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+1 on contamination. Bake at 400 degrees for 20 minutes, re-install after cleaning all of the possible oil or brake fluid from the piston, enjoy.

FWIW the Shimano rotors really do shed heat a lot better, and reduce the temporary noises/loss of power that comes from overheated brakes.
 

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Discussion Starter · #19 · (Edited)
Sure looks like contamination. Push the pistons back into the caliper, wrap a bleed block in tissue paper, and fasten it in the caliper. Pull the lever a few times, then secure it with a rubber band over night to check for leaks.

I think we have a culprit............

I read all your insightful posts this morning and before I left for work I dashed out to the shed and pulled the pads out, ragged the piston faces clean and jammed my bleeding block in with a fold of copy paper. This below is the result after 11 hours.

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I assume this is oil residue on the piston facings? I did run an cleaning rag though the caliper slot and in the direct moring sunlight the ceramic faces looked pretty clean. I photographed one of the piston faces after removing the block and paper - the ceramic face looks wet with oil to me.

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This does indeed really surprise me as I've not noticed any oily residue on the back of the pads - but I also admit I was never looking for oil on the back of the pads, only how the front faces had gummed up.

Actually, come to think of it, the rear lever has come back closer to the bars just a tad - a sign of long term oil loss perhaps?

So, assuming 11hrs of paper on pistons with a tell tale ring = leaking piston seal the next question is what to replace the caliper with?

Do I trust Shimano another time around seeing as this has been an undiagnosed dud from day one? Replace these calipers with the current M9020 or Saint M820 4 piston models keeping my current XTR levers which seem to work just fine at least for 9120 straight swap (I know the banjo bolt joints will be compatible at least with M 9120) Or do I downgrade to XT M8020 calipers or full kit with levers or do I venture into the unknown and go with Magura MT5 or MT7 or go with Hope Tech 3 4 pots?
 

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I had a similar issue with my Saint M820s. I eventually traced it to a small bit of fluid leaking around the piston seals. The pistons had brake dust/dirt buildup on them. I pushed each piston out separately and cleaned off as much contamination as I could. It solved my problem. Every time I flush brake fluid at the calipers it is beyond dirty. I suspect that brake dust gets pulled past the piston seals causing this issue.

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