Brakes and rotors

It could be rust that has fallen through either on the front or back. Maybe recheck and clean off what rust you can from the calipers and brackets. Sometimes if pads don't hit correctly it does that as well. You should do a couple of hard stops where no one is around to seat the pads.

Could be that the caliper piston is not self adjusting/retracting-inspect the caliper assy, also could be the type of disc pads being used