Take the brake pads off and try using a file/sandpaper on the contact surface to roughen it up a little as that surface might be hardened with age, or the pads are so new they still have the 'sheen' on them rom being molded. Last resort is new pads.
Koolstop pads - I tried those once, never could get them to stop squealing like a terrified pig and I didn't think they stopped the bicycle any better than generic pads. I replaced them with generic black rubber pads that work fine and don't make noise. YMMV