Hello: When ONLY the upstairs sink or toilet are used, there is a leak that shows on the large cast iron pipe in the basement. When the downstairs bathroom is used, there is NO leak showing on the big cast iron pipe in the basement.

I have removed plaster from the wall upstairs on the bottom, found the cast iron pipe there, checked it, and it is dry.
Then, I removed plaster from the wall downstairs on the bottom, found the cast iron pipe there, and feel no water on the cast iron pipe when running water from the upstairs bathroom sinnk or toilet.

Is it logical to think, then that the leak is between the bottom bathroom toilet and sink surfacing on the main pipe in the basement, rather than some mysterious leak somewhere else in the walls above it?

Yes, I do not understand the maze.
HELP.
Marie

Hi Marie,
Is this a older house with lead and okum poured and caulk joints? The only way that I know to track a leak is to follow it back to its source. So what are your plans after you locate the source? Your leak might be as simple as recaulking a joint. However, we won't know what the problem is until you locate the source of the leak. Let me know if I can help. Good luck, Tom