try the following:
1) strip the calipers down, remove the pads and clean out all the pad grooves and location in the caliper, smear a thin layer of copper grease on the metal back of the pads where they touch the piston and where they rub in the tracks of the calliper also clean the calliper tracks of debris and dust before re-fitting the pads between the piston and pad and also acts as a vibration damper.
The copper grease won't melt under hi temps so DO NOT get it on the brake disc or friction face of the pads, it does however keep the brakes working smoothly and help reduce any squeal as it damps things out
2) As above but fit an anti-squeal shim as well, these are a either stainless steel shim or sticky plastic pad which fits between the piston and pad and also acts as a vibration damper.
3) file a small chamfer on the leading edge of the pads as this encourages the pad to sit squarely on the disc and not point load on one edge
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