I rebuilt my calipers with new pistons, seals, covers etc about 3 years ago. I've had a 'sticking' piston recently and stripping the caliper down it looks like crud and dirt has seeped under the dust cover and caused the piston to rust. So, I'll rebuild and ....... Is there anything I can use, Vaseline, copper slip or whatever to smear inside the dust cover to try to combat the problem.