I doubt it is. It seems to be quite random. Some even under the seal. I suspect caused by some sort of degradation of the brake fluid when hot. Maybe moisture related.

Regarding sealing, the pistons are near perfect and they ride on the square seal. I think that so long as the seal groove is clean, the new seals should seat and seal properly.

I will clean up as best I can. It would be easier if I split the caliper, but don't want to go there. Thinking I might be able to make a hone of some sort. Also maybe soak caliper in brake cleaner or acetone overnight and see if that softens that yellow stuff!

Seal kits not coming until end of week, so have time to clean!

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5