[Lucky]

Originally Posted by SilverSurfer

Drain the plastic one, make sure the metal one is full, then run up to temperature until the fans kick in.
Then when cool enough check the header/metal one is still full or top up if required. Run it like this for a few days then check again.

If it keeps emptying into the expansion tank the first thing I would try is a new pressure cap for the metal header tank.

Thanks matey