Claudia, Did you or your dad try running a snake down the drain? It sounds like he opened the trap but just looked around. A snake might find a real blockage. But, I don't understand how the water from the hose worked if there was a blockage. The vent stack might be the issue, but strange if none of the other bathroom drains are having the same issue.
-- Blue skies. Dan Rossi Carnegie Mellon University. E-Mail: d...@andrew.cmu.edu Tel: (412) 268-9081
