Originally Posted by Brian Carlton

Can you perform one test, please?

Get it good and hot and then bring it home and park it.

The next morning, squeeze the upper radiator hose. Does it feel soft and easy to squeeze or is it quite hard, as though high pressure is in the system?

Now, carefully open the expansion tank cap and observe whether gas forcefully escapes from the cap when you do it. The system might be under a slight vacuum and air might rush into the reservoir. You need to pay careful attention and make a clear determination whether air is rushing out or rushing in.

Report back with result.