I believe the fuel line has air in it, which needs to be pushed thru the lines towards the fuel tanks.
What do you think about installing a siphon bulb squeezer in the line below the fuel filter? Do you think that squeezing the bulb several times might start the siphoning effect downward into the tanks?
It's worth a try. I just bought the $7 pump yesterday, because I'm having to siphon gas from teardrop tank to engines' tanks. I'll hook it up after the filter and see what happens tomorrow. That should be easy enough, and cheaper than rigging up a 12-volt electric fuel pump.
If it works, the siphon bulb becomes a permanent part of my fuel system.