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?
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.
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.
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.