I have a combination tub/shower faucet that's leaking, at a rate of a drop every few seconds. The system has a shower head, tub faucet (with pull-style cutoff knob), shower/tub selector, and hot and cold handles. (All but the shower head shown is shown in the attached image, with handles removed from the hot and cold stems.) The leak comes out of whichever shower or tub faucet that's selected with the selector knob. It's difficult to tell from the temperature of the leaking water whether the hot or cold line exclusively is leaking. (It's a slow leak, and the temperature is...lukewarm?) I can stop the leak by turning off the hot and cold handles (overtightening them), selecting the tub faucet, then pulling the tub flow cutoff knob. One potential clue is that when I turn the shower/tub selector to tub, pull the tub flow cutoff knob, then turn on the hot water while leaving the cold off, hot water comes out of the cold water stem, with enough pressure to travel horizontally a few inches before hitting the tub, and enough flow rate to cause a constant stream of water. The reverse is not true however; I can't get cold water to come out of the hot water stem. I've replaced the rubber O-ring on the end of both hot and cold water stems, and when that didn't work, both stem assemblies themselves (which included another new set of O-rings), with no change in the leak rate. Any thoughts?