300D no heat w/bypassed monovalve, new aux pump

[bzook]

Hello,

I think I have read just about everything there is to read about the w123's no heat issues and have tried most of the fixes.
Here are my symptoms and fix attempts to date:
1983 300D (not Euro) 213k miles. It's never had heat since I bought it.
The fan blows only from windshield defrost vent, and side vents, nothing from the middle. Assuming bad vacuum pods.
From what I've read, when vacuum fails, the blend door defaults to defrost vents.
I have no heat.
Replaced mono valve with junkyard unit. No heat.
I repaired broken wire on monvalve plug. No heat
I replaced monovalve with aftermarket new unit. No heat
I bypassed the monovalve with a brass ball valve. No heat
I replaced the aux pump with bench tested junkyard unit. No heat
I checked for voltage at aux pump, - 12Volts
I checked flow out of aux pump, appeared poor.
I checked flow out of my roomate's 1983CD aux pump, great flow.
Unplugged the aux pump. No heat.
Bench tested the junkyard aux pump again....several months after installing, and it looks to be frozen.
I replaced aux pump with new, and flushed the heater core. Lots of rust, but was able to flush the core in both directions for several minutes.
All hoses feel warm on both sides of the brass valve, and heater core.
Ensured good flow from the cylinder head.
Coolant level is good. Main water pump was replaced last year due to a bad bearing.
I can get luke warm air for about 10 seconds after I've driven the car for awhile, then it goes cold again.
I've also tried two different push button control panels with no luck.
Let's see, any other relevant info...the AC does not work either, although I'm not looking to fix it here in SoCal. But this El Nino is making it seem like I'm back in Indiana this winter so heat/defrost would be nice.
I doubt there's air in the system. I've pulled enough hoses and reattached while running coolant that it would seem any air would be gone.
The engine gets up to just under 80 C in this weather according to the gage.
Could the heater core need more flushing, or maybe a blend door is stuck?
I would lean heater core since I do get some warm air for a few seconds. What am I missing?