Caliper Half Seal
 

For those who need to split the calipers. Here is a source for the caliper half o-ring. I believe they apply to others, but ATE is the application listed. Hopefully its ok to post the link. Pelican does not supply these obscure, but essential, parts. Go to the link at bottom of the page.
https://www.pmbperformance.com/brake-calipers.html

Centric brake parts makes kits that have the caliper half seals. I used them on my Mercedes and 86 chevy cavalier. I have also used their brake pads on a few of my vehicles.

https://centricparts.com/

See post #70

http://www.peachparts.com/shopforum/diesel-discussion/273570-splitting-brake-calipers-5.html

Thanks for the link. I have bookmarked it. I just ordered some BBC rebuild kits from here that seemed like they had the half seals. I will know for sure when they arrive.

If just an O-ring, that sounds like a generic part. I have O-ring kits for both SAE and metric, in nitrile, HBNR, and Viton, so go to those first. The shell O-ring doesn't move, so common nitrile (Buna-N) should be fine.

It's a flat or square ring. I would only use ones that are designated for brake use unless I could verify that they were suitable. EPDM is recommended.. If there are ranges of quality within rubber types, I don't know. For the minimal extra cost, that concern is taken care of.
I figured getting them delivered was more efficient than calling or driving around town finding a match. A couple of local brake places said they would try, and probably had them. That seemed like a possible runaround, and it's not urgent.
The local hydraulic place did not have the right type. I ordered a few extra because the plan is to redo all the calipers on two vehicles. Although, that has been sidetracked for a bit. I think I paid 35$ with shipping for like 15 of them. IMO, the shipping is worth it to save the hassle.
Apparently I found that link on the thread mentioned above. I wasn't sure if it was from another pelican section. Redundancy is good insurance. Shipping from them was a bit slow though. Once again, it didn't matter.