Maybe you could get someone else to listen at the fuel filler for air bubbling through the gas as you blow. Its possible that the pickup has failed in the tank and no longer reaches the fuel. I don't know what the thump would be, I would listen for bubbles.
If the fuel pump is in good shape, it does not need to be primed, it will prime itself. If there is an air leak in the fuel line, the pump will suck air not fuel.
You might try feeding fuel to the pump from a container near the engine with a known good fuel line. Be careful, fires start this way with any mistake.
I might try to fit a pressure gauge to the output of the pump. I would expect to see about 3-5 psi with or without gas from a good pump. If the gauge is inserted in line and the carb is dry, no pressure will develop as it will be bled off by the open needle in the carb.
