1977 gmc pickup fuel gauge problems

I have a 1977 gmc pickup. My fuel gauge has recently started acting up. I have dual gas tanks, and no matter how much gas I put in either tank, the gauge only goes up to about 1/4 tank level. it will stay there until the tank has less than 1/4 tank and then it will start to drop. I have changed the tank switch , but that didn't help. I am thinking maybe there is a short, but can't find any bare wires anywhere. does anyone have any suggestions?

Bennie