She may be dead, Jim!

[Retroman1969]

I appreciate the input from everyone's combined experience on here!
I just talked to the mechanic who is having second thoughts about his head gasket suggestion. In 20 years, he has only replaced one, and it was from a Boxster that dumped all its coolant and the owner drove it hot. He wants to pressure test everything and see what he comes up with. I suggested a faulty new tank and he paused and sighed. Then said its a definite possibility, but he didn't want to think about it because of the enormous pain they are to replace.
So you guys are on the right track. More likely a tank or cap than a head gasket issue.

[thom4782]

My guess is that the cracked head is a low probability scenario for the same reason you cite.

I recently had my coolant tank replaced is it took a couple of weeks for the system to flush out all the bubbles before the coolant level stabilized. During that time I worried constantly that fluid was disappearing into a cylinder through a crack.

BTW: let me ask the obvious. Are you 100% sure the bleed valve is closed.