The last two times I did this I used a dremel with a small wire wheel on it to clean that area between the piston seal and the dust cap that gets crusty and causes sticking.
The wire wheel did not survive long, and it was kind of a pain to get the pieces all out of the bore, but it got the rust and gunk off quickly. There is a guy near me who breaks the caliper every time and reuses the square orings. He says he has never had an issue with that, but I don't have the nerve to try it.
It seems the 115 caliper kits often have the sqare orings, but the 123 kits rarely do.

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1985 300 TD 448K
1984 300 TD 278K

1983 240D euro 240k
1994 f-250 idi turbo 330K
1986 f-350 IDI
1987 F-350 IDI

1985 JD 1050 4wd
1965 IH 3660