Hmm. Since my owners manual shows the ABS pumps to be linked to one electronic controller (on page 21), I guess I assumed the brake lines were linked, as well. Should have paid more attention to what I was reading. Duh! So, this is good news. Now, I can just focus on the front system.
How much brake fluid are we talking about to purge the pump and calipers? My rear master cylinder was replaced within the past year at BMW. They may have changed the fluid in the front at the same time. It looks clean.