In the heat of the moment, some builders forget to think about the possibility of malfunctions when installing plumbing -- and you may be the one to suffer the consequences. An access door behind the bathtub allows you to service the faucet, but if the builder forgot to install one, your only recourse is to cut into the wall to make repairs. Fortunately, that usually isn't necessary, because most bathtub faucet leaks can be fixed from the tub side of the wall. The fittings may be old and rusty, but there are various tools to help you with that.
Turn off the water to the bathtub before disassembling the faucet. If you can't find a dedicated valve for the bathroom, turn off the main water valve for the house. Open the bathtub faucet to let remaining water drain.
Pry the decorative caps off of the faucet handles with a flat-head screwdriver. If your faucet has a single, lever-style handle, you won't find a cap, because the screw that holds the handle is located under the lever.
Unscrew the screws holding the handle with a Phillips screwdriver or hex wrench. If the screws are rusty, spray them once or twice with spray lubricant and wait for five minutes, and then try to unscrew them with a long-handled screwdriver. Carefully push on the screwdriver as hard as you can before turning the screw counterclockwise to avoid stripping the screw head.
Use a screw extractor to remove a stripped screw. Attach the extractor to a drill, set the drill in reverse and push the extractor into the head of the screw while you run the drill. If the extractor slips on the screw head, drill a 1/8-inch pilot hole in the screw head with a metal boring bit.
Pull the handles off. If they are stuck, you probably won't be able to do this by hand. Use a handle puller, available at a plumbing supply outlet.
Remove the collars holding the hot and cold water valves by unscrewing the retaining collars with adjustable pliers. You may need to soak the threads with spray lubricant to get the collars to turn.
Replace the adjustable pliers with a long-handled pipe wrench if the collars won't turn. Be sure to wrap a rubber glove around each collar before gripping it with pliers or a wrench to avoid damaging the finish.
Use a valve puller to extract the valves. You can buy this tool at plumbing supply outlets. It grips the valve stem and tightens around the base of the faucet, and when you turn the handle with a wrench, it extracts the valve in the same way that a corkscrew extracts a cork.
Remove all rubber parts from the valves, and soak the valves overnight in white vinegar to dissolve mineral deposits. Replace all the rubber parts with new ones.
Clean out the insides of the valve housings with a wire brush. Put your finger in the housing and feel along the valve seat. If it feels rough, remove the seat with a seat wrench and soak it in vinegar, or, if it's chipped or gouged, replace it with a new one.
Set the valves back into the housings and reassemble the faucet. Turn on the water and test the valves.