We are in a 35 yr old, 5 story condo complex. We repeatedly find that mostly horizontal runs of pipe show rust along the bottom of these pipes, indicating cracks. What are the most common reasons for so many failures in these horizontal runs? Also, once these leaks appear, is it wise to do a professional patch of these cracks or is replacement the only proper solution? Is it ever appropriate for a plumber to suggest replacing 5 floors of CI waste line replacement when a single leak is detected? I recognize that in an ideal world, where money is no object, all piping should probably be replaced periodically to eliminate any chance of leaks.... thanks for your advice... Gary 