5kt rear caliper o-rings

[Ameer Antar]

So I've been trying to rebuild my rear calipers for some time now. I bought
rebuilt ones and had them painted. While trying in vain to get the o-ring
behind the piston rod (the one with the acme-thread) to seat, I took it out
and compared it to the seal from the other caliper. It looks much larger
than the other one, so I decided to take an old caliper apart to see if I
could scavenge from that. The old caliper was OE and looks like it was still
original: never rebuilt. That seal was actually in between the two sizes in
the rebuilt calipers. It definitely seems "just right" in the bore, but I
don't think want to use a seal that old and hardened.

The question is: what are my options? Can I order spare seals for these
calipers from Lucas or some distributor? Would I be better off buying new
o-rings from say McMaster? In that case, what am I looking for?
Size/material? Anyone ever done this?

TIA.

-Ameer