To be clear, I was bleeding a fresh master cylinder before attaching the brake lines. This was the master cylinder with all three ports dumping back into the reservoir. There was fluid in the forward compartment as far as I could see. The front port just wouldn't flow enough to bleed properly. Adding resistance to the rear ports got the front port flowing.

IIRC the FSM has a procedure for bench bleeding the master cylinder. It's not clear in the FSM whether a pressure bleeder will take care of a dry master cylinder, particularly in this car without ABS.

Sixto
83 300SD