They tend to ROT from the inside out, if your tube is a older as built in service one for years you would be amazed how thin and spindly they are, even just loosening the retention nut can twist a tube out of shape.
Amen, I've removed a bunch of tubes over the years and found this statement to be true. Salvaged a few, you really have to be careful as the fittings seize on the tube itself and lock up - when you get the threads free in the manifold, you can spiral a tube quickly if it won't break loose from the corrosion bond between the tube and the nut. Kroil or PB Blaster soaked liberally beforehand help, slow and careful moves work best.
You can repair the tubes if you are handy with a TIG or MIG welder - source the proper pre-flared pipe end (brake lines) from your local parts store, find a smaller tube that slips inside the cut and sectioned pipe to support the tube internally, very slight chamfer to each side to allow weld penetration, TIG or MIG stitch on the lowest heat setting until you have the joint filled, and you're set to grind off the excess and coat the pipe. You'd be surprised how effective it is.
After all that, a better suggestion is to buy the best reproduction you can while they are still available. The '67-'68 small blocks are gone, nobody does them anymore - likely indicative of the shrinking market. '69 small and BB are the only first gens left, who knows how long they'll last.
Regards,
Steve
