I just discovered that one of my tub valves was leaking inside the wall for quite some time, given that the wood floor in the adjacent closet has gone moldy and the drywall beside the tub out past all the tile is soft. It was probably 1 drop every second.
Last night I installed a couple ball valves so I didn't have to leave the whole house turned off while I mess around with this, but I am not 100% what to do next. The leak is coming from some sort of adjustment valve right under the cold water handle. I don't know what this thing does or what it is called so I am having trouble searching for a solution. It is accessible from the tub side through a hole in the valve trim plate, right underneath the handle. See the hole in the photo below. Anybody know what this is and how to repair it before I just start taking it all apart?
The drywall behind the thickset mortar bed tile was totally disintegrated in this area. There was insulation wicking the water all over the place. Without the insulation the leak probably would have just gone down the hole in the slab for the drain. For reference, the house was built in 1977, slab on grade in north Florida.
Update
I pulled the handle off to get a better look. The bottom adjustment piece is where it is leaking from. What does this thing do? I'll need to get a special wrench to open it I guess. My sockets don't fit with the tile there.
Best Answer
If you have drywall in the shower that has failed due to the moisture you need to replace it. If the corner is wet and failed you will likely need to replace the adjacent wall too.
At the very least with access in the back you are looking at installing a new valve. You can troubleshoot yours but for $30 you can get a builder's grade (what you have) or $100 you can get a nice shower/tub valve. This needs to be swapped out and the connections.
After you swap valve I would leave that open for a while. You can inspect the wall and make sure it dried out and you don't have mold growing. Just know that there is a pretty high chance of this wall failing but really no need to do it now as you should be able to install new valve without hurting it.
Note: If you do not want to retile now I would suggest using pex/sharkbite as you can just take them off to install new boards. I would definitely not weld your copper to valve unless you are doing tile at the same time.