bleeding Duo Discus brakes

Looking for advice on bleeding the brakes on my Duo with the tost 30-9
caliper. I replaced a shot bleeder valve with a new stahlbus valve,
but after futzing with the system for over an hour, I can't get the
brake to firm up to an acceptable level.

I've tried using a suction device to pull fluid (Dot 4 in the Duo)
down through the line. I've also gone with my normal old standby of
pushing fluid up through the brake using a syringe. The Stahlbus
valve is not working as advertised or rather the brake isn't. The
brake doesn't seem to be creating enough pressure to overcome the
spring tension on the check valve.

The best I've been able to get to so far is a fairly spongy grip on
the rotor. Enough that turning by hand isn't possible, but it doesn't
feel like it would slow you down much when you needed it.

Basically it just doesn't feel like the master cylinder is drawing
fluid in or maybe there is a hidden air bubble.

Any thoughts from the internet hive on the quirks of this beast?
It's certainly been a much greater hassle than I expected. I'm
guessing at this point that either there is a trick that I don't know
about or something is malfunctioning.

Thanks,

Morgan