New problem with the 1958 gas tank

[54cruzer]

My 1958 Cadillac fuel pump is now sucking air when the fuel tank level is below where the outlet fitting is located. I have replaced tank fittings dozens of times, this one has me stumped. The fitting is about 1/3 of the way up on the corner of the tank(see pic). I have had the tank out(very clean inside), the pick-up tube runs at an angle down towards the center of the tank,where it is welded to the bottom.
I think the problem is the fitting, (different from most tanks?-usually a tapered fitting or an inverted fitting will do the trick?), the pick-up tube is flared inside that outlet and I don't think it is welded(or was it, and it's not now?), it needs a flared fitting that will seal against that tube. The problem is that the threads are 5/16, and they don't make a 5/16 tapered fitting with the same thread, that I can find(a tapered fitting wouldn't seal against the tube anyway)A flare fitting would have nothing to seal against.? I used a nipple fitting that straight threads into it, with a lot of teflon tape to rig the seal, but I know it's not sealing against that tube, where I think it is leaking internally???(no external leaks now) The original line was leaking right at the tank fitting-tube would not tighten up, so I cut it off and installed the above described fitting and hose, it is not leaking externally around that fitting-anymore.
The fuel pump is new, the lines are new to the back. I know there is fuel in the tank, I have drained four gallons when it's at this point. The car runs perfectly until it gets to a 1/4 of a tank, I put an electric pump on it and it sucks air at this point, fuel runs steady when above that point.
Was there a rubber seal inside that fitting? I thought about putting a feral inside that fitting so the tubes could seal together? I thought that maybe there is a pinhole high up on that pick-up or something? A new tank is about $400,if I can find one,  hate to give up on this one.
The place (classic car auto parts) where I got my 55 CDV tank has a lot of cadillac tanks, but skips 57-58.  Does anyone know another source or how to fix this one?

[Steve Lachowetz (No. 25332)]

I had the same problem with mine, except it was leaking, not sucking air.  Thought I was missing a correct fitting.  I finally used a bullet tip fitting with pipe thread that I wrenched down into the tank to seal against the pickup pipe.  Then fitted a compression fitting to it to attach the gas line to.  It stopped my leak, but not sure if it would suck air or not, I never let the tank get that low.  You can see more at:

http://my58caddy.com/page/9/

Steve

[54cruzer]

Does anyone know how the fuel line sealed to that fitting on the tank?  When I unscrewed the factory line(that was leaking and wouldn't tighten up), just llike any other piece of metal fuel line, you could clearly see that there was nothing in the gas tank fitting for the line to seal to?

[Coupe]

Cadillac started using a bushing at fuel outlet in '49. Later AC cars had the same set up on the return line. I'm not sure how long it was used but I do know it was used thru '58. CLC member Jeff Maltby had some machined. I do not know if he has any left. Here are a couple of pistures from his posting selling these bushings.

[54cruzer]

A HA!!  Great info, I thought I was missing something, I guess the last guy lost it, no wonder it was leaking when I got it! thank you!

[54cruzer]

I think it would bubble and hiss out of both, so you wouldn't be able to tell?  It's got to be leaking without that bushing-it seals the two lines together.

[Jeff Maltby 4194]

Car 54. I'm having 6 more made up as soon as the shop can make them. I'll keep you advised.

[Jeff Maltby 4194]

Kevin. Machine shop said they will have one out by this friday ? Sorry for the wait.