Quote:
Originally Posted by Early_S_Man
The OEM pistons are hard chrome plated, and from your comment [and obvious concern] about brake fluid not 'wetting' the piston surface along the scratch ... it sounds like the plating may have come off during your buffing operation. It probably won't give you any leaking problem near-term, but how long are you willing to drive the car with 'worries' in the background whenever you step on the brake pedal???
The piston was probably scratched during disassembly for a previous rebuild by a grain of sand imbedded at the chamfer of the caliper bore. Compressed air can move that piston rather rapidly!
This problem illustrates why silicone grease should always be used during caliper rebuilds! It does not run or migrate once installed. A 'bead' of silicone grease at the bore chamfer/piston interface acts as a barrier against entry of dirt or sand, even if the rubber dust shield is cracked or torn away!
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This is good advise. Use only silicone grease designed for brake system rubber seals - that is a silicone grease that does not cause the rubber to swell. Brake rubber seals are made of EPDM or EP rubber.