Regarding Don Barton's note about repairing pot metal flanges and
elbows, I have always had good luck simply coating the inside and
outside surfaces with either PVC-E or <gasp!> Elmer's glue, _provided_
that the piece is still structurally sound.  This seals the surface
and seems to prevent further deterioration.

Elbows that have broken at miter joints can sometimes be successfully
repaired with high-strength epoxies (JB Weld comes to mind), then
sealed, and will typically serve well if there are minimal forces
involved in reinstalling it.  It works for me; your mileage may vary.

John Grant