Dual fuel tanks

I have a 1988 f350 diesel both tanks had about an 1/8, stopped put 3/4 in the front tank, drove for a while looked and had 1/2 on the guage, I switch to the back tank and the guage went past empty and tried to cut off ,switched back to the front. Came home and cut off in driveway wouldn't start. changed fuel filter loosen lines to bleed, charged batteries. Was hard to start but started up, warmed up and changed oil and filter, started up and let idle for a while, went up the road 1/8 mile started slowing down and cut off ran batteries down ,pulled home. Next day I went and got some fuel in a can (5 gal) put in back tank and checked guage ( front had 1/2, back had 1/8) wouldn"t start, charged batteries. Next day checked fuel guage front had 3/4, back was below empty. I'm asking why is the fuel leaving the back tank, and is it sucking air and wont start. Thanks