I have to tell you, I am a little red faced here. I have lived with this problem now for over a year. I would simply open the heater control valve and let the core fill, then close it and turn on the fan to warm the cabin then open the valve filling the core then closing it. If I left it open the sucker would overheat. You could literally watch the needle going up!
So yesterday I am adjusting the brakes and what not around the car checking fluids and I notice that the temp sending unit is literally two inches from the heater control valve in the head. Anyway, I just got to thinking about this problem and decided to vacuum out the system again. The rad level was about two inches down from the cap, it's a big old tank so this left plenty of coolent in there. I know I did it before and I know the heater control valve was open (I'm pretty sure I did that, it's cooling systems 101) so I vacuumed it out and left it go maybe longer, really making sure. The rad hoses were collapsed good. All the while no coolent was sucked out by the vacuuming! There normally would be a little bit being the coolent was so high in the rad. Most any time there is coolent at all in the rad you are going to get a little being sucked up. Anyway, after vacuuming it sucked in a bunch of coolent! maybe a half gallon or more. I left it there under vacuum for a lot longer than usual, it will get down to a few pounds and I often will just pull the thing off and top off the tank and that's it. But this time I left it there until the gauge read zero while I worked on other things on the car.
Today on the way to work I drove all the way with the heater on! It ran perfectly normal the whole way, it would have never done that before. There WAS an air pocket in the heater core!
Damn that vacuum system is awesome, do bad I didn't do it right a year ago.