Michael;

I have recently taken apart Calipers on my 122, which I rebuilt about 30 years ago, with new OE Pistons, after which they were in service, or at least in contact with DOT5 silicon brake fluid while parked most of that time. The Pistons and Caliper internals had plenty of accumulated rust particulate (from I expect the insides of the original steel hard lines), but there was ZERO rust on pistons or bore IDs, and all rubber (Seals and Dust Boots) were still in excellent (reusable!) condition! I attribute this impressive long-term result to the use of non-hygroscopic DOT5 fluid.

I will be rebuilding my Master Cyl as a part of a complete brake overhaul, and be replacing and flushing all new (non-corroding) KuNiFer hard lines and (non-occluding) teflon/SS lines, so I wont have the need to flush out non-compatible brake fluid, but when I first converted over to DOT5 fluid, I pushed one full Master reservoir through each corner (at least once), until it came out clean and without drops of dissimilar fluid, figuring that had "flushed" out all the old fluid.

This is first-hand experience, and no particular recommendation for you, but I think that addresses your questions, and I know where I stand based on this experience.

Cheers