I'm leaning towards fuel tank getting air. How to I fix the fuel tank switch that's a common leak point?

 

The quick and dirty way is to find the lines going to the old tank, cut them with enough length to work with them.
Then spray a bunch of something like WD40 down the tubes. I mean a bunch, you want to soak the inside of that unused side of the valve. Maybe do a couple treatments, on both lines. Then cap them off good. You may need some rubber fuel line that slips over the nylon stock stuff and then plug the rubber line. Or I guess you could kink the **** out of the nylon line and then zip-tie it with two or three ties. Anything that clamps permanently will work.
Anyway do this to both hoses, pickup and return. Then go cycle your fuel selector valve like 100 times. Or just switch it 10-20 times every day at work/home for a week before you start the engine.
THIS IS KEY. I had a "bad" fuel selector valve, and took it off the truck for 3 years. Just out of curiosity I opened it up on the bench, saw nothing broken, put it back together and with 12v cycled it. It would get stuck not quite switching all the way. I lubed it up a bit, and cycled it a bunch until it would seat all the way. Re-installed it on the pickup, have been using it a few years ever since. All it took was some lubrication and cycling it many times, nothing fancy.
This is a common issue with the valve, but an easy fix.

However if that doesn't work, or you'd rather bypass it altogether:
Not gonna lie, It's a PITA to get to. It lays between the driver's frame rail and the rear fuel tank, which means you have to drop the rear tank to get to it. You'd think it would be easier to just cut it out and run some new fuel lines but there isn't much space for your hands. Give it a look and see if you can do it. Otherwise you'd have to drop the tank.
I think I managed to bypass it once a long time ago with the tank in, but it would have been easier in hindsight to drop the tank I think.
And if you drop the tank, replace the fuel pickup in the tank with submersible fuel line and then you can run it almost all the way empty and your gauge will work properly. There are a ton of threads about it already.

AAAAANNNDDD, if you're dropping the tank, consider getting a bronco 38gal tank. I have one and it's AWESOME. Cause i've been in your shoes with the single tank and busted pickup tube, and it's a real pain. Now with the bronco tank I make around 450 miles on just that tank, switch to the front and get over 600 before I fill up again.

 

Oh and if you choose to by bass the valve you need to pinch the nylon lines with something or you will take a diesel shower. NOT FUN.

 

This is my 1st post; '91 F250, 7.3. First was dying at 1/4 tank, I assumed fuel filter and changed it. Now rear tank did it at almost 1/2 tank, always while running down the road. Carefully filled tank to max with 13.5 gal.; most I have ever gotten in there has been less than 18 so gauge may be off a bit. No problem with full tank.
My son says there is a strainer at the end of the fuel pickup in the tank that sometimes falls off leaving a gap that sucks air. He says he has heard owners will just add an extension with a small notch or two so's it won't suck itself to the bottom of the tank. Is this what you meant about "replace the fuel pickup in the tank with submersible fuel line"? Heavy enough to lay on the bottom of the tank? Any particular Bronco 38gal tank?
My mechanic says there is supposed to be an electric fuel pickup 'module' at the end of the pickup tube but he can't find it in his parts listing.
My problem is different than 3000gtman since it is with a low tank level and only while running, like running out of fuel. Bleeding the Schraeder valve has always worked but only for a short time. Seems like a lot of air. Mid tank was less than 1/4 but didn't fail this time; in fact I used it to get out of traffic.

 

next time it wont start after only forty minuets poor water over the IP approx. one gallon, are you giving it some throttle when you start it hot? unless its extremely cold out it should jump to life when restarted after 30 or 40 min of being shut off. you may have a worn injector pump. do not rip it out until you have confirmed several times that it will start only after you cool the IP.

 

And don't pour very fast/use too cold of water. They say the thermal shock can crack the IP housing.