Possibly Glazed Rotors?2 methods I use:1. Get up to crusing speed - brake very hard down to about 30, repeat 3 or 4 times.If that doesn't work2. Use crocus cloth and "sand" the rotors using cross-hatch motion to remove the glaze.
My experience with squeal has been from the back of the pads. A vibration occurs that makes the noise. At the auto parts they have some stuff that looks like goop. Pull the pads and apply to the back side, let dry and reinstall. Works for me.
Avoid the over spray cleaning products that gets on the rotors. I use Windex with Vinegar to cure the squeals for a decent time period, but others say such is a snipe hunt. Last set of pads I installed with the anti squeal backing material and it helped, but the squeal did come back after several months. The vinegar based windex stopped it again.
Since it's a used bike, I have no idea of what was done to it before. I will try sanding the rotors first then if that doesn't work, I'll get some "goop" for the back of the pads. It has new tires, maybe I'll just buy a new set of pads and make a fresh start.
Go to AutoZone and get a can of Brake Quiet. It is a tacky spray that you apply to the back of the pads. Pop out the pads and spray the back with the Brake Quiet and put them back in. This stops the vibration of the pads, which is what makes them squeck.
I apply this to all brakes when new and have never had a problem. It will also work for used pads.
When i purchased my wing new the front brakes were squeling to i just cleaned the rotors after each ride mostly used mild cleaners not sure if this helped but over time the squel went away i replaced the front pads a while ago and the squel has not come back yet