bathtub drain leak: how do I know if it is something OTHER than bad putty?

Hey guys... quick question: I've got a tub leak that I noticed last night. Today I filled the tub up full and let it sit for 4 hrs to test if there was a leak around the outside rim of the drain. Not a single drop came through the leak spot in the room below during this time. As soon as I started draining the full tub, the leak started and was fairly strong. The leak stopped a few minutes after the tub was drained. I've never fixed one of these before... but from what I understand, the full tub test is actually testing the putty seal, is it not? If the putty was bad, it would've leaked the whole time, no? If I am wrong, that is good news since it is an easy fix (just get new putty). If my thinking is on the right track, then what do I need to do to fix this leak? I'm assuming it has something to do with one of the pipes' connection to the drain.

Any help is much appreciated.

Thoughts? Suggestions?

 

Yes, it sounds like the seal between the drain and tub is good and the problem is in the plumbing outside the tub. It's time to cut open the ceiling to get access to the piping under the tub drain area. Without knowing how old the house or tub installation is it's hard to say what you will need. It could be a cracked joint or fitting or as simple as a nut needs tightening. I think the worst part of the job will be repairing the ceiling when you are done.