Carbureted Fuel System Help?

A couple of months ago, my truck starting running really rough: hard to start and wouldn't idle. After some diagnostics, ChatGPT informed me that the fuel pressure from the mechanical pump was too high, and eventually ruined the seat/needle my Quadrajet. I replaced the main seat and needle; I put in a high-flow 0.135 main seat. I also installed a QFT regulator in-line between the mechanical fuel pump and the carburetor and have it set to 5.5-6.0 right now. I have two fuel filters in-line, one right before the pump, and one between the regulator and carb. The system is running and working good, but the big issue I'm having now is the line from the pump to the carb is draining out after sitting overnight.

The top fuel filter is empty, the bottom filter is full. The mechanical pump has 3 lines (inlet, outlet, vapor return), so I'm not sure if it's draining back through there, or if something else is happening. (I know that Quadrajets are notorious for leaking from the bottom plugs, but when I press the throttle, the accelerator pump shoots fuel, so I'm pretty sure the carb bowl still has gas in it and isn't leaking.)

After the line empties, I have to crank the starter for several seconds multiple times before the engine fires. I'm worried I'm going to ruin my starter. Also, the mechanical fuel pump died last week and was replaced (not sure if that is related or not.)

I tried installing a fuel check valve in line between the pump and regulator, but that didn't help at all.

So, I need some help figuring out the fuel system issues: why the line keeps draining, why it would run for a few years with no regulator or anything, and then start having issues (seat/needle, regulator, pump failure, etc.), Do I even need a regulator and if so, what pressure should I set it at? The Chilton manual says 8 psi, ChatGPT says 5.5-6.0 psi, and a local shop near me says 4.0-5.0 psi.