I'm having trouble with a leaky shower. From what we can tell by where the water drips it is coming from right behind the fixures. The only time it leaks is ashen the shower has been on for about 10 minutes. Doesn't leak when the bath is running or draining EXCEPT if the bath is running and you stop the faucet the leak will start. So it seems there is some seal or washer that the shower and bath use that only leaks under pressure. Any ideas where the leak might be?