Generally speaking, brake squealing is caused by contamination of pads or disk. If you had squealing and replaced the pads and still have squealing, I'd first think that your disk might have been contaminated and just recontaminated the new pad and the disk stayed contaminated itself. This would be evident by black rings on the disk. First step I personally would do is bake the pads in a preheated oven at 400 degrees for ~15 minutes or so and lightly sand the surface by rubbing the pad on the sand paper(on a flat surface) in a circular motion until the surface looks clean. Then do the same with the disks and then clean with isopropyl alcohol on a rag. You'll have to "bed in" the pads after that process. This is all assuming they aren't new brakes. If new brakes, you might just need to bed in the rear pads to quiet them down.