I have noticed that squeal is usuallu due to oil on disc. People think that not touching the braking surface with a finger or hand is no biggie but it is, it really is.

fortunately there is a ridiculously easy solution for oiled discs and pads. Clean the disc with pure ethanol (not denatured, that leaves a residue) or some other fully vaporizing degreasing agent (brake cleaner works as well). Pop the pads in the oven for half an hour at 392 fahrenheit. Assemble and now you have a non squealy brake system with renewed power.

I did that with my mtb and nearly otb'd at first braking due to the new found power