Here's the problem I have. I have one new tank on the left side 13gal tank, put in about 4 years ago. The right tank is an original tank from 1995 time period 13 gal tank.

This is what is happening. I put 5 gal gas in the right tank and it goes dry, meaning the light for low fuel comes on and the header tank starts to go empty, not good. It seems to work if it's full. This gives me the indication that the unusable fuel is 5 gal. If I bank and pull back on the stick, fuel flows again. Not something I like to do. The left tank works all the way down to almost nothing in it, no matter what I do flying wise.

The right tank(troubled tank) has the vent in it at the top for the header. I've tried making the tabs for the cap tighter for a better seal and I have the blue seal from a mercedies fuel cap. I also have the inlet on the cap flared so it catches the airstream, still no good.

This tank picks up from the very back corner. I just sealed that and drilled a new hole twards the bottom center like my new tank, but I haven't had the chance to see if it makes things better. Almost seems like the tank is not pressurizing with the vent in the cap, but I can't see anything wrong with the cap, can blow through it and can seal it up by plugging the inside of the cap.

Any and all thought appreciated.