Afraid of the answer.....

[911monty]

Originally Posted by robdelorenzo

Thanks.
Ordering a new cap then.

Well It's cheap so yeah, but I'm only giving thought to things to check. The purge valve can also leak in this same area and was also exposed to the meltdown and could now be compromised. You will want to remove that plastic cover over the expansion tank and attempt to identify the leak point. As an FYI the cap is very easy to cross thread.