HISSIN50,
I'm pretty sure the caliper pins are coated with Cadmium (haven't checked on the SEM

and this acts as a lubricant so the calipers don't jam on the pins. The first time I changed mine the coating was gone and the calipers would hang and cause uneven wear. So now I just put new ones on with each pad change and lube them with caliper lube.
Don