Pulled apart the rear calipers on my '88 Porsche 924s yesterday, with the hope of rebuilding them (having never done that before). Questions for the hive mind - is this piston ok to reuse as is, can it be polished to remove the rust (if so, with what), or am I better off just replacing the caliper?
This is what the caliper looked like coming off the car:
That is fine. Use a 3M green scotch-brite pad or similar and it will be like new.
I've wet-sanded brake pistons by putting them in wet-use sandpaper under a dribbling faucet and then turning the piston in the sandpaper until it looks good, with good results. Use a piece of sandpaper that can cover the whole height of the piston surface at once and use even pressure.
I'd use a very fine steel wool on that piston. Should be fine.
The seals do all the work so the condition of the cylinder in the caliper is more important.
If you can scotchbrite the piston (I like WD40 as a fluid for this, $22/gallon at Autozone) and there is no flaked off chrome where the seal tracks, it's good to go.
Make sure there is no rust buildup in the caliper between the dust boot and seal. This is a fairly unimportant area, you can clean that up with scrapers or die grinders or whatever, just so long as nothing can grab the piston and keep it from moving.
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