If the pickup is on the bottom and you're still not able to get fuel I think you're 'sucking wind'
i.e. there is a hole in the line.
Mechanical pumps do take a long time to prime.
I often fill my bowls with a syringe through the vents sticking up in the mouth of the carb.
This has always proved enough idle time for the timing case mounted pump to get fuel from the tank to the carb.
Jim,
Lil'Red is a '87 F250 HD, 4.10's, 1356 4x4, Zf-5, 3G, PMGR, Saginaw PS, desmogged with a Holley 80508 and Performer intake.
Too much other stuff to mention.