I have water leaking down the main stack and have found that the source of the leak is the top (second) floor sink and shower. The toilet is not contributing to the leak. The stack is exposed in the basement, but is behind plaster walls in the kitchen and second floor bathroom. I’m not sure where to start breaking things out. Where does it make sense to make holes; the kitchen ceiling, kitchen wall, or bathroom floor? Any ideas? How would a professional plumber approach this problem?