Fuel pump issue

Hi everybody! I'm having an issue with my fuel pump and I'm hoping someone can help me out. I have a 1951 Pacemaker sedan with the stock 232 six. The mechanical fuel pump is an AC dual action (fuel & vacuum). I also have a cheap 6 volt electric pump mounted near the gas tank which I use as a primer pump to push the gas to the carburetor when starting. Once the car is started I switch off the electric pump and run on the mechanical pump.

A few months ago I was returning from a road trip and I noticed that I had left the electric pump on. I turned off the switch and a few moments later the car started to buck and stall. I switched the electric pump back on and the car smoothed out and ran fine the rest of the trip.

I assumed the mechanical pump was bad and had it replaced, but I'm still having the same problem. The car seems to be starved for gas and it will stall out if I try to run on the mechanical pump alone.

The only thing I can think of is that the mechanical pump does not have enough strength to pull the gas through the electric pump, but I've had this setup for several years and never had a problem before. Anyone have any suggestions? Thanks!