Worn seals, rubber washers and gaskets in the valve assembly are the most common causes of leaks in tub faucets. Replacing these parts can solve most of the problems with tub faucets.
Ensure that you have new parts available before you begin to repair your faucet. Replacement parts are available at home centers and hardware stores. Carry your old parts to the store to get a proper match.
Before starting any work on your tub, establish the water shut off and turn off the water to your house. Drain any water inside the pipes by turning on the water in the tub.
Dissembling the faucet handle is the most difficult part in dissembling the faucet parts; over time, corrosion virtually welds the handle to the stem. Locate the screw on the faucet handle. It may be exposed, or you may need to pry the center off the faucet handle. Using a special handle puller makes the work easier.
Sometimes leaks occur because the seat washer stiffens over time and does not seal tightly. Replacing the seat may fix a leaky faucet. If your faucet is two-handled, replace the seats and washers in both the cold and hot valves.
Before replacing the stem parts, lubricate them with plumbers grease.