Shimano brakes squealing - please help, out of ideas!

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CaMKii

Posted: Jul 9, 2021 at 9:16

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Set up:
Calipers: Shimano XT M765
Rotors: RT64 (new as of late-May)
Pads: New J04C metal pads (previous pads were same compound)

Problem: Did a fresh bleed with fluid change and put in new pads. Cleaned rotors with isopropanol during the bleed. The first couple of rides were ok, but then brakes started squealing loudly and doesn't seem to be getting better. I've never had this before - usually in the past the pads bed in after 1 ride, and offer quiet (dry) performance for the life of the pad.

Steps taken to try to solve the problem:

1) Re-cleaned rotors with isopropanol
2) Re-cleaned pads with isopropanol
3) Sanded down top layer of pad
4) Realigning caliper by loosening bolts, squeeze lever, tightening bolts

I don't believe there is contamination - despite the noise, braking power feels normal. But I don't know where the resonance and vibrations would be coming from as I'm using the same method of aligning the caliper as I always have.

Anyone have any other suggestions I can try?

eshew

Posted: Jul 9, 2021 at 10:44

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It is contamination.

Your best bet will be to heat the pads thoroughly, ideally with a propane torch.

I have done this plenty of times and it works 1st try 95% of the time.

Remove pads

Heat with a propane torch for 45 seconds per pad, with the flame directly on the braking material. Careful they get hot! Would be a good idea to wear eye protection just in case.

Let cool, for a while. Don't burn your fingers

Gently sand down the face with a clean piece of sand paper

Clean rotors with alcohol

Reinstall pads.

Break in pads, find a hill or pedal hard with teh brakes on. Focus on the front or back individually. It takes a few minutes to get everything bedded in & to re-season the rotors.

CaMKii

Posted: Jul 9, 2021 at 11:10

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eshew wrote:

What convinces you that things are contaminated? Not saying I disagree but wondering if there's a "smoking gun"? I also bled my front at the same time and no issues with noise.

I've been bleeding my brakes for years and never had this issue. I also cleaned the rotor and all around the caliper when I did the bleed, and since the noise started I've cleaned the rotor and cleaned/sanded pads three times now. Only thing I haven't tried is the heat.

CaMKii

Posted: Jul 9, 2021 at 11:10	Quote
duplicate post deleted.

CaMKii

Posted: Jul 9, 2021 at 11:11	Quote
duplicate post deleted.

iffy

Posted: Jul 9, 2021 at 11:22	Quote
try manual alignment by eye sometimes things shift when you tighten the bolts, and check to see if the disk is being pulled to one side in a few places, might have some dust pushed into the seal with the piston causing one to be "lazy"

CaMKii

Posted: Jul 9, 2021 at 12:03

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iffy wrote:

try manual alignment by eye sometimes things shift when you tighten the bolts,

and check to see if the disk is being pulled to one side in a few places, might have some dust pushed into the seal with the piston causing one to be "lazy"

By eye, rotor is centered in the pad clearance, both pads look to be moving well when lever is squeezed.

iffy

Posted: Jul 9, 2021 at 13:22

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I would scuff up the disks again with wet n dry, clean everything and bed them in again, shimanos have been known to have issues with leaks around the seals, some internet wisdom is they can spray fluid mist out under braking, not really visible. Also there are small o rings between the 2 caliper halves which some people have replaced and it cured this random issue

the fact it was quiet for a while sounds like some kind of contamination

scrap that the 765 is one piece I think, pretty old, likely the seals are likely perishing if original?

BenLow2019

Posted: Jul 9, 2021 at 15:00

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Maybe try the realignment again, but this time put a piece of paper between the pads and rotor.

This is a little tough to juggle and may be a tight fit depending on the type of paper. I use two small slips of paper that used to be the back of a big sticker so the paper is really slick on one side.
Use one piece for each side of the rotor at the same time.
Loosen the caliper, slide the papers between the pads and the rotor on each side.
Wiggle the caliper a little and then squeeze the lever a couple times.
Make sure the paper has full contact on the pads when you do this.
While the lever is pulled, tighten the caliper bolts.
Remove the papers and put them away for another day.

This method gives me nice results every time.

Hope it helps.

eshew

Posted: Jul 11, 2021 at 10:02

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I'm my experience with Shimano brakes any time things get squealy and/or braking power diminishes I assume it's contamination. The propane torch heating method is my go to solution and it's always worked after 1 heating session outside of once when the pads were overly contaminated. That requires 2 heating sessions.

It's fast, easy and works.

Since you still have good braking performance I'd guess you did a good job getting rid of most of it but not all.

Assuming you've already made sure bolts are tight and rotor is centered contamination is the most obvious explanation for your symptoms. Somehow something got on the pads, sometimes it's from a messy bleed, sometimes it's from a perfect meticulously clean bleed, chain oil or just weeks of being off the bike.

[/Quote]

What convinces you that things are contaminated? Not saying I disagree but wondering if there's a "smoking gun"? I also bled my front at the same time and no issues with noise.

I've been bleeding my brakes for years and never had this issue. I also cleaned the rotor and all around the caliper when I did the bleed, and since the noise started I've cleaned the rotor and cleaned/sanded pads three times now. Only thing I haven't tried is the heat.[/Quote]

tbartoncv

Posted: Jul 11, 2021 at 11:47

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Sounds like contamination. I’ve had a few m785 calipers leak at the o-ring between the two halves of the calipers. It doesn’t take much mineral oil to make it squeak. I’ve also had leakage at the pistons. m785s are a few versions old now and seals are getting long in the tooth. I had success with this kit:

https://www.amazon.com/Bicycle-hydraulic-maintenance-XTRM9020-calipers/dp/B08HD94YKC/ref=asc_df_B08HD94YKC

danielson-01

Posted: Jul 29, 2021 at 9:38	Quote
Most things have been mentioned already: 1: Grind disk and pad 2. blowtorch disk and pad 3. center / line up brake caliper to disc 4. if still squealing, slightly grind down the edges of the brake pads (this is how lt‘s solved on most cars of the breaks squeal)

GlenHammerLT1

Posted: May 20, 2022 at 21:31

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I have the exact same problem..
I have been searching the web trying to find a solution to my problem with brake squeal.
Exact same rotors as you have had the problem with but MT BR420 calipers.
Back brakes perfect, front brakes no matter how many times I sand/clean rotor or clean or even replace pads, they glaze up across the centre.
I think it is the rotor as the surface seems to be not perfectly flat but raised where holes have been stamped through on the outer side of the stamp, if that makes sense?
Did you solve the problem at all?

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