If the in-tank pump is working, (nor running, but "working") then no priming, purging, no messing around with anything... Drain the original filters, replace them with what you're replacing them with (unfilled, zero cross contamination, zero mess, zero of the "balancing a filled filter without making a mess" game. When the filter(s) are in place, flip the master switch on and leave it on, open the bleeder at the top of the secondary housing a quarter turn (regardless of what filter was changed), and when it's done purging air, close it. You are done.
If it's not that easy, either you are doing it the hard way, the transfer pump's not working or not working correctly (the in-tank pump) or there is a stopped line between the tank and that point which must be corrected.
Start with the secondary bleeder. Remove the screw entirely if you want, or just loosen it, depending on how much evidence you need to see to be convinced. Is there flow?
If not, remove the supply line at the tank, adapt a clear hose that you can run up to a visible location and back down to the fuel tank filler cap. Is there flow?
This should either lead you to an answer, or allow you to move on to another system besides the supply section.
