[Blue Bowtie]

A three-line fuel pump should be able to draw fuel "uphill" from the tank, but it may need to get the check valves wet with some fuel before it can do it consistenly. The marine primer bulb is not a bad idea, but none of those were ever rated for any heat, so it should only be temporary and then be removed so the original line configuration can be reconnected.

Have you tried blowing backward through the fuel pickup line to the tank (with the fuel cap removed) to see if the pickup sock filter in the tank is plugged with sediment?