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.
Before removing any tile, what is on the back side of the wall in question...
If it is just drywall, it might be easier to just cut that and do any repairs through the back....
I've been considering that. It backs up to the drywall by the side of the toilet in the other bathroom. Not a lot of room there, but would probably be easier than working on the tile.
It could be a bad tub spout seal.
Nomix Inc. repair cartridges that fit Mixet brand valves
It is a Moen style stem to replace the Mickey Mouse one that Mixet uses.