Pot metal or lead type elbows fail because of the different metals
used in the old alloys.  They will continue to deteriorate even
when plated as the metals used in the casting process will continue 
to chemically react in a bad way for elbows.  I believe is called
"intergranular corrosion".

What sizes are most common?  Currently when I need one I make them
up with a flange soldered to a brass elbow like the ones used by
Wurlitzer.  Or so I have been told.

Don Teach - Shreveport Music Co.
Shreveport, Louisiana