I took the expansion tank cap off this morning, and lo and behold, it blows
and it sucks!
I assume that it should be pretty difficult to suck air through it , which it
was not.  I have ordered a new cap (no one in Washington DC stocks it, go
figure).

Is this my problem?  Or is the thing designed to let air out easily and only
gives resistance to liquid?

Thanks for the advice,
Peter and Zippy