There is a check valve in the return fuel line. Mine was in the return line that runs from the filter down to a fitting by the fender. I replaced that line and discovered the check valve, it was INSIDE the hose by the lower fitting, about the size of a empty 22 shell casing. Truth is I found it by accident and had no idea what it was so...I dropped in my pocket (tg) installed my new hose and merrily headed for town 30 miles away. About the time I got there the engine was missing shuddering and died. Hard to start and when it did it didn't run long. A mechanic friend came to my rescue, checked everything and never did figure it out. I ended up copsing it home. 4 hours later I got home found the check valve and installed it. The car ran great!
I'm stupid for not putting it back in the line, but I'm glad I kept it because when i try-ed to get a new one, nobody had ever seen anything like that! I wondered if it was just on my 83 sport?
Oh if you drive for a while and there isn't pressure when you open the gas tank, you probably need a check valve on your return gas line, and don't put it in backwards.