Repair a shower mixer valve by removing the pressure-balancing unit, flushing the lines and installing a new unit. The project takes about one to two hours and the cost of parts is $75 and up, as of April 2015.
Remove the protective cap from the handle by prying gently with a utility knife blade. Remove the screw to that holds the handle in place and the handle. Take the two screws that hold the retaining plate out, and remove the plate. At this point, two screws that are the water shutoff valves are visible on either side of the valve. Turn them to the right before proceeding.
Remove the brass screws that hold the front of the assembly in place to access the cartridge, and use pliers to pull the cartridge from the assembly. Clear the water supply lines by placing a paper plate under the valve to direct water into the tub, opening and closing each of the shutoff valves.
Install the o-rings in the new cartridge and the cartridge into the existing unit, ensuring the keyhole at its back matches the key in the housing. Apply plumber's grease to the large o-ring on the front of the housing, press it into place and insert the retaining screws. Turn the water valves to the left to the On position. Replace the faceplate, handle and cap.