I have a real puzzler - my cast iron bathtub (36 years old) has a small leak that I can not find the source of. I have access to the bottom of the tub around the drain and can see up to the spout/shower riser area. There is a small amount of water leaking through the tub bottom approx. 3.5 in to the side of the drain. I have verified that the water is not running from another point and accumulating at this spot. I have also verified that there is no visible crack or hole in the porcelain of the tub at that spot (or another other spot for that matter). The leak only shows up when someone is showering - any suggestions?