Sooo...
This ain't working.
I'm convinced there's air in the combination valve/ABS that's trapped and I can't bleed out.

The service manual states to bleed the combination valve after the master cylinder and before proceeding to the wheels- but nowhere is the procedure to do so in the service manual and I've used every online search engine as well. The top brake line is higher than the reservoir so it's logical this can be a problem if the reservoir runs completely dry.
There is what looks like a pressure relief valve of some sort at the top of the combination valve under the black rubber cap (see photo). As I mentioned previously, perhaps I also need a scanner that can activate/bleed the ABS module?

Anyone?

 

You need to be able to bleed air out at all four wheels. Did you possibly get the right front brake bleeder screw issue repaired?

If so, you can try letting gravity bleed your brakes. This process can be slow, but it works. You can open the rear right bleeder screw first and watch until you see a steady stream of brake fluid drip at the bleeder screw. Close that bleeder screw and move to the left rear and do the same procedure, all while checking the brake master cylinder to ensure it stays full of fluid.

Another option is to perform a pressure bleed of the brake system. You can get a pressure bleeder that attaches to the master cylinder to pump brake fluid under pressure into the brake system. Once the pressure bleeder is setup, it's a matter of opening each bleeder screw to bleed off the air until you see a good stream of brake fluid.

Lastly, you can use a Mityvac brake bleeder to pull brake fluid through each bleeder screw, while checking the brake master cylinder to ensure the fluid level stays up.

 

^^^
Kept at it another hour plus.
I kept getting an air bubble at the very start of each bleed cycle after cracking the left rear bleeder, which didn't seem "right". I noticed there was more play than I expected after loosening just a quarter turn or so to bleed.
Suspected that it was sucking some air through the threads- so I unscrewed the bleeder, plugged the hole with a silicone plug temporarily while I wrapped the bleeder threads with teflon tape and then screwed it back into the brake cylinder. Seems to have solved that problem, don't see air any longer but pedal still going to the floor.

Perhaps sitting for a few days dry after leaking out, or the repeated bleeding damaged a seal- I'm going to replace the master cylinder tomorrow as I'm outta ideas. I didn't figure on the master going bad at the same time as the cylinder blowing out, but perhaps that's it. Just my luck that the usually simple to diagnose brake issues just ain't been so...

Thanks to all for the feedback. Hopefully a new master will wrap this up.

 

Here are a few tricks I've picked up over the last 6 decades. They are in no particular order.

I bleed by myself by getting some clear plastic hose that fits snug on the bleeder. Loop it up and over something so the bend is ABOVE the bleeder. Run the hose down to a bottle. I like the clear maple syrup bottles as they have a decorative handle that is perfect for wire to hold it in place. (Plus, real Maple syrup is yummy!) Put a little brake fluid in the bottom so air doesn't get sucked back. Now just pump slowly. Check your master and keep it full.

I use a small brass machinists hammer to tap the steel brake line at bends. Sometimes an air bubble will catch and fluid will flow around it. This doesn't happen often, but it can.

Start at the right rear and do all the corners. Even if you've bled them already.

If the bleeder is broken, replace the unit. Wheel cylinders are shared across brands and models. Unless you have something odd and rare, like a Deusenburg, you can usually get a replacement. I might not be cheap, but you probably can.

Clean, clean clean. I use NON-chlorinated brake cleaner to make sure EVERY friction surface is clean and smooth. Chlorinated works too but I like my liver and want to keep it.

Descale your drums and paint them. Your choice of color. Rust flakes hold heat more than clean steel. I use high heat engine paint.

If the fluid does run out of the master, fill it and gently tap the brake pedal. You'll see some bubbles pop up. Let it sit a bit then repeat. Bubbles in the line will slowly roll up. I've bled clutch masters like this too. Gravity is our friend as fluid is heavier than air.

 

So process of elimination worked.
FINALLY got the right front bleeder freed up, more kroil, some shots from the impact hammer, and vise grips with a screwdriver through them for a bit of leverage and it broke free. New bleeder screw (with teflon tape), bled the right front, then the left front (again) and...still the same problem.

So I was down to trapped air in the ABS, but the pedal was still way worse than "spongy". Felt exactly like it did before I replaced the master about a year ago, and given it was so recent I really wasn't thinking this was the "new" problem but it was $50 and a couple of hours to get it, bleed and install it.

Brakes are now perfect. All I can think is that the constant bleeding and pressing the pedal to floor wore out a seal(s) at some point- and it went from being spongy from air after replacing the rear wheel cylinder to fluid bypassing the seals in the master cylinder.

Go figure...