thanks for the responses-
Drove the car last night with a full tank. no purging. this happens irregularly.

I checked the gasket in the cap and the lugs previously. everything looks right and the cap closes securely. I know it seals because there was a whoosh sound when I opened it the first time yesterday. I think the tank had a slight vacuum. I've cleaned the overflow tube before but don't see how it could effect the tank volume, pressure, or vaccuum.

the tank collapsing is an interesting thought but I don't know where the vaccum would come from unless the return line is blocked and the tank is collapsing as fuel is drawn out. I'm going to apply air pressure to the return line today and listen for air / bubbling at the filler opening.

The day it lost 3-4 gallons it started on 3/4 full and by the time I got home, it was at 1/4. I drove a distance of 115 miles smelling fuel the whole way literally watching the guage drop.

Any other thoughts are appreciated.

Rgds