I also had an experience attaching a nipple to a tracker bar.  The
problem started when I was removing the old tubing and getting it
prepared for new tubing.  One nipple broke off.  Apparently the nipple
was not too-well soldered originally.

Not being a soldering expert, I was very skeptical about using solder
and applying heat to the old tracker bar.  I did not want to cause more
damage from excess heat.

So I used metal epoxy:  J.B. Weld, or something similar to it.  I mixed
the two resins, put a small amount on the tracker bar, and also the
nipple.  I worked it in place to its original position, held it in
place with a rubber band, ran a pipe cleaner through the nipple to be
sure no epoxy was inside of it.  The epoxy hardened, and the nipple was
perfectly attached.

To reinforce it, I gave the area a second coating of epoxy.  There have
been no problems and the player has been in use nearly 20 years since
this was done.

Bruce Clark