Little help with cooling system please.

Good you didn't pull the heads. People jump into that too often without checking first. It is normal to have a little air space at the top. If not, the cooling system corrects, as yours did. I have a "combustion gas leak check kit" I got years ago. Cal Vans brand I recall. I recently read about a new coolant that changes color if it sees combustion gases, to warn you. I was searching for "waterless coolants".