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I have cleaned the brakes because the pistons were getting a little stuck.

I’ve also cleaned the (resin) pads with isopropyl alcohol and the rotors too. I then went down a hill slightly braking to have the pads “bite” and brake again.

It was all good for about a day and then it started squealing while having very reduced braking power.

I cleaned rotors and pads again, sanded the pads down a little with fine sandpaper and everything was fine again for a day, then squealing.

When I took the pads out, they had developed mirror shiny streaks across them. I replaced the pads with new ones and everything seems fine now.

I’d like to understand what happened to these pads, why are they so noisy and why are they developing these mirror like streaks?

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    Oil was probably "hiding" in the vent holes of your rotor. It's hard to get it all off a totally saturated disc rotor, so don't worry. The heat of braking will loosen it up and help it to migrate back to the braking surface. It's a shame you ruined another set of pads but it's easily done.
    – Noise
    Commented May 2 at 8:56
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    a hint: slighty braking downhill is not a very good way to bed in your new (or cleaned) brake pads. If i remember correctly, Shimano recommends braking hard from ~30 km/h 30 times, to bed them in. Dragging the brakes to heat them up can overheat them, resulting in glassing.
    – Burki
    Commented May 2 at 10:08

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Lessons learned: organic solvents are bad for braking pads.

You can use whatever you want on the rotor, isopropyl is (imho) OK for the braking mechanism itself.

Pads are to be cleaned with hard brush, if at all needed.

Pads are OK-ish with water (this is how they survive bad weather), but water won't make them any better either.


How solvents degrade brake pads? Braking is rather complex proces, but here is an ad-hoc idea: they dissolve some soluble component and migrate it to the surface on heating, filling in the surface irregularities. This is how the glassy spots happen.

The glassy spot has the ability to grab the rotor at once, build up tension as the rotor continues to rotate and then release it at once when the tension overcomes the strength of the sticky effect. Many times per second. This is how the sound is generated.

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    There is no generally acceptable way to clean pads once contaminated with oil. The oil permeates the friction material too easily.
    – Noise
    Commented May 2 at 18:49
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    Pads are consumables - best to just replace them once the rotor is completely cleaned.
    – Criggie
    Commented Jun 1 at 21:17
  • Oh jeez, go buy some CRC brakleen and use it if you contaminated your disk brake pads with oil. Use automotive disk brake anti squeal. Disk brakes are nothing new under the sun and automotive engineers solved these issues decades ago. Commented Jun 2 at 13:36

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