could be a weak pump and you can do a pressure test, could be the float setting, but the bowl isn't supposed to be "full" [like a pressurized system], there's a proper level setting .... it doesn't "drain back to the tank", pretty normal for most of these old carbs to leak and evaporate the fuel out of the vented carb float chamber if they sit for even a couple days, it could be you're just not running the truck enough
Bill
