No the clunk should not be able to go to the front of the tank. If the clunk line bends and sticks in the front it will give you more problems. Went I set my tanks up I put a piece of brass line inside the clunk line so it can't bend forward ever.

I'd check to see if it isn't stuck forward all ready. Can you even see your tank?

I think you have some other problem besides the cluck, even if you only had small amount of fuel in the tank and you nosed down there's still fuel in the line, you couldn't nose down long enough to run out. If your getting air bubbles in your line its probably because theres a small pin hole in a line or the tank itself or a problem with the stopper cap.

What kind of plane is it?
How is the tank mounted?
Is it high or low in relation to the needle vavle?

-Steve