I'm installing a new shower valve for a new bathroom in our house. I've plumbed everything in, and I can't get rid of a tiny leak on the bottom of the shower valve. Since it's just a shower, I plugged the threaded tub filler connection with the supplied brass plug. No matter what I do, I end up with a leak of about 1 drop per 2 hours. I've got a pipe wrench on this sucker and have tried 3 wraps / 6 wraps / 8 wraps of teflon tape, but it leaks every time. Is there some trick i don't know about? My other threaded connections are leak-free.
As someone suggested, check the cast. If it is a crack in the cast, you may be able to make it reveal itself by grabbing the body of the fixture and applying pressure in various directions, like you're trying to bend it.
But it still could be leaking through the threaded joint. That would be my first guess. Is the plug tight? Does it get tighter as it goes in? Does it bottom out before it gets tight?
You may want to try a different plug made from galvanized or make one from copper using a male fitting, a small piece of pipe and a cap. (That's what I did on a recent shower project.)
By the way, I've always had much better luck sealing joints for the long haul with teflon tape than pipe dope. I've had pipe doped joints start leaking months later. I hate it when that happens.
Edit: By the way, there shouldn't be any pressure on that joint when the water is turned off, so it should leak faster when the shower is turned on, if that's where it's coming from. Is that the case? If not, the water may be coming from one of the pressurized joints and migrating to the plug. Also check the joint for the pipe that goes up to the shower head. '
Is this a traditional two-handle set-up?