Sorry for the late update. It arrived yesterday (Genuine GM Part). Nothing changed. Today I also loosed the water pump drain bolt, until I got a good stream of coolant out (thinking there may be trapped air in there). The reservoir dropped a bit so I topped off. Started engine and same issue arose again.

 

Pretty sure at this point that you have a bad head gasket. I don't like being a bearer of bad news, trust me.

 

Yea - looking that way. I am going to run a few more test tomorrow and if there is no other progress I will start working on the head gasket.

 

Go with a Fel-Pro head gasket. Best made gaskets around for a very long time.