Calipers aren’t cheap – even re-conditioned ones. They get rusty of course – unless you follow the trend for covering them in a shade of red Dulux Gloss..and you can get caliper repair kits. A certain amount of rust close to where the piston dust cover inserts around the piston is inevitable..with time, and salt, this corrosion can gradually creep closer to the piston seal. None of this will effect the response of the caliper piston to braking pressure. To reduce the corrosion you could put grease around the bottom of the dust boot to stem the tide of rust.Which grease? By know you know not to use a petroleum based grease which attacks elastomer rubber..it swells and it’s ability to keep water out is then reduced as it gets loose around the top of the piston. You could use ProSlip PIN — keep it for it’s intended use though and use silicone grease instead.
When there is rust under the seal -change the caliper. You could remove the rust under the seal…but since water got in there in the first place the horse has bolted – corrosion around the base of the dust cover means the cover no longer keeps water out. Rep
see the post ‘Piston Seal – Into the Groove’ – for a fuller discussion on why rust under the piston seal is bad news..