I've got a Mixet shower/tub control that is about 25 years old. We've developed a leak inside the wall that comes out from under the tub. It leaks only when the water is turned on. Closing the valve stops the leak.

I'm thinking I probably have a loose joint or cracked copper pipe going to the tub spout; however, I wanted to check if there was any incident of these valves leaking only when open. It'd sure be a lot easier if I only had to replace the cartridge rather than removing tile and tearing out the wall.