header tank query

[smokeyone]

I almost know the answer is yes but to double check should the header/expansion tank cap on my rr 3.9 make an airtight seal.
If I squeeze one of the cooling return pipes I can hear air escaping from
the cap. Also, not certain but think the top radiator return hose is empty of water because it squeezes much too easily and does not feel solid if you know what I mean..

Thanks for any advice..