Chevy van dies going uphill

I have a 1979 chevy g10 sportvan with a 305 engine. Some time ago the van started to cough and die when going up a hill. It would not start at all while on the slope but if I rolled it back to a level surface it would start right up. I found that if i filled the gas tank up, the problem would go away. I figured that i had a problem with the sending unit, maybe the tube that runs toward the bottom of the tank was corroded in some way so that it would only suck gas when the tank was full to a certain point. So i kept the tank at least half full with no problems. Then the problem returned when the tank was 3/4 of the way full. I figured that the sending unit was getting worse. I removed the gas tank and removed the sending unit which looked fine. I was disappointed.
I began to drive my farm truck around and the van sat for awhile- maybe 6 months. My farm truck finally died, so back to trying to get the van running.
I put a new sending unit in since i had the tank out anyway, put about 5 gallons of gas in it and headed for the nearest hill. Seemed to work just fine. Problem solved, right? After about 100 miles of driving the problem is back again. I figured o.k. the carb float must be getting stuck, so i have removed the carb. i found that the fuel filter was crushed/ installed improperly, it was just kind of smashed and twisted looking when i took it out of its housing. Yes i am the one who did that and yes i am an idiot. I cannot exactly remember if the problem existed before i changed the filter last time, but i suspect that it did. I have the carb apart right now, it certainly needs a cleaning, but the float did not seem to be hung up at all. The van has a mechanical fuel pump. I imagine if its not pumping enough pressure it could cause it to stall or bog on a hill, but it won’t start at all on a slope after it dies. It starts immediately on the flat.

I guess my questions are these:
Could the filter problem have caused the stalling problem?
What else in the fuel system could be failing due to gravity?
Should I be looking elsewhere for the problem?

I think that this is still the old mechanical fuel pump. These normally either work or they don’t.

The first thing I would check is that the breather tube has not been plugged by something. This tube has to allow air to enter the tank as fuel is consumed. If it is plugged, the pump will create a vacuum in the tank and eventually the pump cannot suck more out.


Have you changed the fuel pump? I remember back in the days when my cars had mechanical fuel pumps the first sign that the pump was failing was that the car would lose power on hills due to the diaphragm stretching and the fact that the fuel pump, when going up hills, is more elevated being on the engine.