I can't really tell what the fitting or the problem is from the
photo. It looks as if it may be in a solar heater. The solder
looks to be gobbed on the outside of the pipe cap, not drawn into
the joint. The drop tube that was capped was probably silver
soldered to the main tube. Is it the cap that is leaking or the
down tube joint?
Sometimes things get so crudded up, it is beneficial to just
replace a section of pipe and get back to some native pipe in good
shape. The rust and dirt pattern almost look like the main pipe
or down tube joint is leaking. The joint needs to be hot enough
to draw solder into the joint, not lay on the outside. Have you
ever removed the cap completely? If you can remove the cap and
you can see where either the cap or the pipe is not fully tinned.
If they are completely tinned and properly heated, they will seal
as long as there is ABSOLUTELY no water making steam in the joint
area. If you can't remove the cap, there is a chance it was
silver soldered, a totally different process that requires a
totally different solution.
It looks as if the clearances are extremely tight. I would try to
get the incoming large pipe cut loose, enlarge the entry hole at
the frame to see about getting a bit of wiggle room at the top of
the frame. If you could get the main tube cleaned up and cut, you
could bring in a fresh piece of pipe with a coupling and eliminate
the by-passed tube. I don't think you will ever get enough
clearance to use a tubing cutter, think jig saw or Dremel. Once
soldered in, waterproofing the enlarged hole in the frame should
be fairly easy.
Keep the whole world singing . . . .
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