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How to fix a bathtub faucet that leaks only when the shower runs?
Our bathtub faucet leaks a steady stream of water. Sometimes this will temporarily slow to a drip if I am a little more firm in tightening the valves or switch the water flow from faucet to shower head but it eventually comes back.
Here are some pictures to illustrate the setup:


I saw a wonderful response here: How to fix a bathtub faucet that leaks only when the shower runs? It's not quite the same but I can see the similarities. I'm still confused though.
- Do the symptoms I'm experiencing suggest a faulty valve? If not, how should I troubleshoot?
- Is it possible to replace the valve with minimal damage to the wall? I'm wary of having to tear down the tile/wall to get access to area.
- Since this is likely a very old setup (40-50 years old) are there any "gotchas" I need to be careful for? Are parts going to be harder to find? Will I need special tools (in addition to the Shower Valve Socket Wrench)?