Thanks for the ideas.
I just rechecked for an empty or low rear reservoir. I can see fluid sloshing against the top all over the reservoir, so I think the rear is filled.
Thanks if6was9 for the clue to finding the splitter. I'll check it over the weekend. While I'm under the back I will reapply the vacuum pump to see if the vacuum holds with the bleeders open. I believe they did when I tried before, 97_SL320. I didn't know NAPA stocked parts for this vintage, okyourabeast, thanks for those tips.