Tub mixing valves contain a removable cartridge.
Most single-handle tub faucets with temperature control contain a removable cartridge that regulates the flow of water. While this cartridge can break, it more often becomes clogged with mineral deposits and sediment and requires cleaning. If the faucet leaks, the rubber seals around the cartridge and water inlets may also be worn and require replacement. Does this Spark an idea?
Remove the Handle
Turn off the water to the faucet by shutting off the valve under the house on the line that supplies the bathroom. If your house lacks this valve, shut off the main water supply to the house. Next, remove the handle from the faucet. If the faucet has a round plastic handle, you can do this by by popping off the cover plate and removing a screw. If the faucet has a metal handle, the screw is probably under the handle, out of sight. You will probably need an Allen wrench to remove it.
Remove the Cartridge
A circular piece of plastic may be attached to the valve stem; if so, you'll need to unscrew and remove this before you can access the cartridge, which may be held in place by a nut or a pin. Unscrew the nut with channel-locking pliers, or remove the pin by gripping it with needle-nose pliers and pulling it straight out. The cartridge can prove difficult to remove. You can buy a special cartridge puller to do this, but a cheaper way is to reattach the handle and use it to pull the cartridge straight out. Make a note of the orientation of the cartridge in the valve seat; you will have to put it back in the same way.
Service the Cartridge and Rubber Seals
Examine the cartridge for signs of wear or breakage, and replace it if you find any. If the cartridge is just dirty, you can usually clean most sediment with a high-pressure water spray, such as the jet from your garden hose. You can also make a strong solution of tub and tile cleaner and water, in which you can soak the cartridge to dissolve heavy mineral deposits. You may have to scrape some deposits off with a slot screwdriver or awl; if you do this, take care not to damage any plastic parts.
Examine the O-rings around the cartridge and the rubber seals in the water inlets, and replace any of these that have become worn or excessively dirty. Remove the seals from the inlet holes by prying them off with a screwdriver, and use the screwdriver to push them back into place.
Reassemble the Faucet
Once all the seals are sound and you have cleaned the cartridge, push it back into the valve seat in the same orientation it had when you took it out. Drop the pin back in place with needle-nose pliers. You shouldn't have to force it if everything is correctly aligned. If the cartridge is held by a nut, screw this back on and tighten it with channel-locking pliers. Finally, screw the plastic part back onto the faucet stem, if there was one, then replace the handle and turn the water back on.
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