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.
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).
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 is the reverse of removal.
