I strongly suggest that you contact you Attorney to discuss the problem. What I can say is usually a problem within the walls is something the building super can repair and this should fall squarely on the shoulders of he building. If this problem occured due to on overflowing bathtub, it would be on you alone.
In short, you are responsible for any problems within the apartment. The building is responsible for anything that occurs in the building. Good luck. I'd love to know how this pans out.