Help with bowed neck

[Uncle J]

Recently aquired a vintage bass with a Fender style neck (flat headstock). The bass has for some years sat unstrung, and the neck has a a strong downward bow. I've restrung the bass, but have not been able to get the tension on the strings taught enough to keep them from touching the 12th fret. I've also raised the saddles on the bridge to see if that might help, but it hasn't. It's been a few days since the bass has been restrung, and I've added tension to the stings each day hoping to raise the action, with very little change. Also, the trussrod nut ( which is at the body end of the neck) has been completely loosend, to allow the neck to bend upward.
How can I get an upward bow on the neck (get the neck to flatten out)?
All suggestions will be grealty appreciated.
Thanks, Uncle J

[J.D.B.]

Is the neck actually back-bowed? You didn't mention whether you put a straight-edge on it. if the neck isn't back-bowed, check to see that your neck pocket isn't angled backward, that is, the surface against the back of the neck should be parallel to the top of the body. I have a couple "polished turds" with poorly machined neck pockets. I shimmed the necks forward a bit and was able to get them set up nicely. If the neck is, indeed in back-bow then, I can only guess that a forward-shim and set-up may well get the neck to loosen, over time, into (hopefully) "controlled relief". Worse comes to worse, you could likely replace the neck.
Hope this helps ya,

Josh

[Uncle J]

I've checked the neck with a straightedge resting on the 7th & 8th frets. There is slightly more than a 1/4 inch of space showing at both ends of the fretboard. I'd like to keep the original neck if possible. Hoping for an easy solution, but I'll probably remove the neck and try to straighten it by using a clamping fixture.
Thanks for your reply. Uncle J