How to fix a bathtub faucet that leaks only when the shower runs?

Question

I just moved into an apartment and the faucet in the tub is fine when all of the water is off. It leaks when I turn on the water and switch it to the shower head. What could be the problem? It's a rental so I don't want to take the switching mechanism off to investigate. I suspect it's some sort of rubber seal in the switch.

UPDATE

The control is a twist knob between the cold and hot twist knobs.

Answer 1

Sounds like the washers/gaskets on the valve have gone bad, you'll have to either replace the washer or the entire valve (if you can find a replacement).

Here is what the valve may have looked like when it was new (just to give you an idea of what you're dealing with).

Removal

• Turn off the water to the shower faucet using either a local shutoff valve or the main water valve for the house.

• Turn the shower’s hot and cold water all the way on, to drain leftover water from the pipes.

• Pry the cap out of the center of the diverter’s handle.

• Remove the screw in the middle of the handle, and then pull the handle off the faucet.

• Unscrew the metal sleeve that sits over the valve stem.

• Insert a Shower Valve Socket Wrench over the valve stem and turn the dirervter anti-clockwise to remove it.

• Replace the washer, or the whole diverter if you can find a replacement.

• Apply some Silicone grease (plumbers grease) to the washer before replacing the diverter.

Installation

Installation is the reverse of removal.

• Insert a Shower Valve Socket Wrench over the valve stem and turn the dirervter clockwise to install it.
• Screw on the metal sleeve that sits over the valve stem.
• Push the handle on the faucet, then replace the screw in the middle of the handle.
• Push the cap into of the center of the diverter’s handle.
• Turn the shower’s hot and cold water all the way off.
• Turn on the water to the shower faucet using either a local shutoff valve or the main water valve for the house.