All good suggestions - checking for condition of rubber lines, fuel pump pushrod condition etc. if they are all good maybe this below should be considered.

Maybe just disconnect the fuel line from the pump outlet up to the vapor canister and crank the engine and see if you get pump output. If so, then do the same at the canister output. Possibly the return line to the tank from the canister is allowing too much fuel return after setting. Aftermarket return orifices appear to be too large in some repop examples. The production canister is pretty small.

Another test would be to then pinch off the return line to the tank and see if the line to the carb gets primed.

On my car I did change the vapor canister as suggested, when I did that the old one had orifice way too big, so I replaced it with a proper one, I tought that has to be the problem, but no it did not do anything to my situation. Also to be clear when I put fuel in the center carb I'm putting it in through the vent pipe directly to the bowl, I'm not spraying it down the center of the carb. I will try that to see if that works, I have a syringe with fuel and inject down that tube.
In my case I don't see any signs of leakage anywhere on the outside of the carb.
This year I will look at the push rod and now look at the hoses.

I have the same problem with my sixpack. If it sits more than 3 weeks, I have to pump the pedal a lot while cranking like I'm flooding it. It think the gas is evaporating. Today's gas really sucks.

I fought that a couple of years ago on my 440-6. You could crank for ever and it would not start. Pull the shaker set up off (big pita) and give it a drink of fuel and it would fire right up. Come to find out the center carb pump discharge needle valve was sticking in the seat. The gas had left enough gum build up that it would stick after sitting for more than about 6-8 hours. And these carbs had just been freshened up a few months prior. Cleaned up the seat and valve good and everything was fine. The gasoline is the biggest problem we fight today!

Hey, Just run a carter 4070 electric pump at the tank, and still use the factory fuel pump. I have used this set up for 30 years racing and in a 79 Power wagon for plowing. Works beautifully! The factory fuel pump acts like regulator kind of sort of....but the 4070 Carter doesn't need a regulator. I have same loss of fuel issue on my 87 Dodge W-150 with 440 in it and old school AVS. The hot motors evaporate the crappy gas.
What a joke.

^^^ I've heard multiple people share the same, that the 4070 in tandem works great.

So how would you plumb that carter electric pump in, do you bypass the mechanical pump on startup then shut it down or leave it on.