After three thousand miles the front brake on my son's adult tricycle developed a loud squeal. I disassembled the hub and all looks about as it did when it was new. Plenty of material on the shoes/pads. Braking power hasn't changed. The squeal happens at very low speeds, right near the stop. My son is autistic so it drives him up the wall. I can see no grease or oil leakage. I have ruffed up the shoe surface with sand paper, this helped for about a block and a half. I can't find replacement shoes from SA. I guess I could replace the front hub but that seems a bit much. Any suggestions?