We have a ball valve in the fuel line right next to the filters. I only use it when we are changing filters in the hope of not loosing prime. Otherwise, we have not had any problem. Anyway, I was once advised to close that ball valve if we were going to be parked for a bit with the front on a serious downward incline. So one question would be about whether you are on flat ground. In any case, it could be useful to install such a cutoff valve and see if it helped with you current problem. If it did, you would know that the fault is forward of the valve. If it did not, you would know that the problem is behind it. It would also serve as a safety when changing filters.
