Don, could be that you are getting exhaust gases into your coolant.  These
come past the gasket at the top of the cylinder.  The reason you keep getting
air out of the bleeder and not any coolant is that since you have made the
radiator the hight spot in the system the exhaust gases migrate there and
leave the system.  Since there is a continual flow of gases into the coolant
the "air" never completely gets out of the system.
This is my theory anyway.
Solution:  pull the heads and get new gaskets (or rebuilt heads too if they
are found to be badly pitted upon inspection).
Ken Wilford
Van-Again
John 3:16