Chevrolet Repair: will not start unless primed, chevy silverado, turning the key

Question
97 Chevy Silverado, 350 Vortec.  Will not start unless primed. Runs fine after starting. Will not restart immediately after turning it off unless primed again (little gas or start fluid).  Thought it might be ecm. Purchased a used one.  Does the same.

What might it be?

Answer
The available fuel in the gas line sounds like it is returning to the tank and losing its initial priming capabilities.
Does the fuel pump kick on when you turn the key to the on position? If yes, try turning the key to the on position then turn the key to the off position then back to on again and try starting it (all quickly). Let me know if it starts.