Problem solved!
The large O-ring in the coolant bleeder valve was bad. There is a strange crimp in it that is likely from initial assembly, I don't think it has ever been apart. It is unbelievable that it ever provided a proper seal. The fuzz in the picture is from a rag.
It was a 15 minute, $5 job to remove the valve and replace both O-rings. Now my tank holds pressure and the coolant level only moves from mid-tank to 3/4 tank from cold to hot.