I'm trying to seal an open galvanized drain line (1930s era) that's
embedded in my masonry walls. The line is about 6 inches in the wall,
and rather difficult to get to.

There aren't any threads left (all broken or rusted off) to screw a
cap in.

What do I do at this point? I tried a test-plug and I still get some
leakage around the edges.

Any advice?

-Matt