W126 (300SEL) Odd brake problem after bleeding

[BrettS]

I was doing a fluid replacement in my brake system (1989 300SEL). I inadvertently let the rear-brake reservoir run dry.

When I topped it off again, everything bled through just fine, but I had a bit of a spongy pedal.

Assuming I didn't bleed it enough to get all of the air out, I re-bled. This time I kept getting bubbles out of the right rear. Some were large, but mostly they were numerous and small.

As we continued to bleed, my assistant working the pedal said they pedal got suddenly firm (though the RR bleeder valve was open at the time. It was reported that there were two pumps with a firm pedal, and then it went soft again, and remained that way.

After the two firm pumps, both rear lines had significantly diminished fluid flow (per pump of the pedal).

I closed off the bleeder, checked the fluid level (which was fine), and tried the pedal myself. It was soft, but could be pumped up. But when pressure was held it would slowly sink to the floor. On the next attempt, the pedal stopped resisting at all, and just went right to the floor (couldn't be pumped up).

I assumed something had gone wrong with the master cylinder. I pulled both brake lines from the master cylinder and inserted plugs. The pedal, with plugs in place, was very firm and held its position.

So, the master cylinder appears to be okay. There are no apparent external leaks (and the fluid remains at the same level if pumping the pedal with the bleeders closed).

Is it possible that it has something to do with the ABS module? (I don't know anything about how that works.)

Brett