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I recently found a bicycle used by a relative which seemed to be in good shape. However, I found that the hydraulic disk brakes make a loud squealing sound towards the end of the stopping distance. After watching videos on the same problem, I've tried sanding the pads, Sanding down the disks and even bleeding the brakes, in that order. Nothing has worked. Just today, I bought new brake pads and installed them but... The squealing is still there!

I don't think the disks are out of alignment becuase they don't make the typical periodic noise of misaligned/ bent disks.

What should I try? Should I just wait for the new pads to break in?

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I am a professional Bike Mechanic. Your rotors should be cleaned with isopropyl alcohol (or automotive brake cleaner if you must) and nothing else. Any sort of lubrication on the rotors including WD40 will cause them to squeal.

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You might try a different brake pad compound.

I had a similar experience using organic pads. Switching to a semi-metallic compound pad eliminated the slow speed grabbing and most of the noise. See what pad compound you are running and try a different compound pad.

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It can also happen if the disks or pads are not totally free of degreaser (e.g. DW40). Not sure you've considered it already but it is worth mentioning it.

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    Did you mean WD-40?
    – Paul H
    Commented Aug 15, 2022 at 3:15
  • LOL yes indeed ^^
    – julien-ottawa
    Commented Aug 15, 2022 at 11:20
  • I've never used it in my brakes. Plus even if there was some residue, it would have been removed with the sanding.
    – mahela007
    Commented Aug 16, 2022 at 4:07
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    WD-40 provides some lubrication and is very unsuitable for use on brakes
    – Noise
    Commented Aug 16, 2022 at 9:42
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After about two days if riding, the noise seems to have settled with the new brake pads. Maybe they needed to be broken in. I will post more updates.

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    What kind of new brake pads did you get? I have a similar problem (mainly when the road is wet) and I've tried out the other suggestions from the similar questions without a result. So I'm very curious to find out new things to try.
    – Saaru Lindestøkke
    Commented Aug 17, 2022 at 20:10
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    Glad to hear that the new pads quieted down quickly. Disc brake pads can take some time to "bed" in to the rotor. It is kind of a break-in process, bedding is, and it applies to automotive disc brakes as well.
    – Ted Hohl
    Commented Aug 18, 2022 at 2:48
  • @SaaruLindestøkke the cheapest brakes I could find. Things liken brake pads aren't readily available in my country. The box said they were "sintered" pads
    – mahela007
    Commented Aug 18, 2022 at 7:03
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My brakes started to produce this noise when the pads were simply worn. My commuting at the time included a long hill that required brakes on descent, so they lasted for less than a year. The noise went away as soon as the pads have been replaced. The braking quality was not degraded yet at that point.

It were hydraulic disk brakes but twenty years ago I had another bicycle, rim brakes but very comparable hill. In a year the pads very obviously needed replacement when the rest of the bicycle still looked and worked almost as new.

Hence depending on where and how the bicycle was used, the brake pads may be most worn part.

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