you may have a poorly functioning caliper. what you may want to do is a lube on the pistions with some high temp silicon lubricant (autozone) and do a quick resurface on the pads and disks with some medium grit sand paper. dont forget to clean everything afterward with brake cleaner to get all the grit out. if that dosn't work than you may have to replace that caliper. on your last break set did you have thumping on one side?