Fuel issues.......

I have a 2OO6 78O with a d12. Im having some type of issue but not sure where to start. It only does this when im low on fuel. My gauge will be in the red area far to the left. Not all the way, but my digital gauge will say i have fuel left. Yesterday I had roughly 2O gallons left and i had to shut the right side fuel valves off because it was sucking air. But i go down the road to fill up and i put 85 gallons in the right side which tells me i had 15 to 2O still in there!!! This is the 2nd time this has happened. In the past ive ran it down to where there was only 2-3 gallons left according to my gauge. But now im afraid to run it low....any ideas???

 

For the most part I usually run it low, ive gone as low as 1-2 gallons, but not often and dont make it a habit, but now i cant go below 2O or so, 2 times ive had to shut off right side,,,,,,,, Ill look into the proportioning valve........thanks. Does that valve return it to the passenger side??? Or how does that valve work....?

 

I just read on another thread from a few years ago where you replaced that valve?? Where are my fuel tank vents located???

 

No i look and can see in the tanks that I still have fuel left. Someone who owns a Volvo also mentioned the fuel pump supply? He said it's on top of my spin on fuel filter......