The Moen/Stanadyne cartridge is the part of the shower valve that is designed to open the valve and let the water pass through the pipe into the shower head. The cartridge can become corroded or worn out to the point that it no longer turns properly. It can also develop deposits on the cartridge that prevent the water from shutting off completely. Removing the Stanadyne cartridge can be done in about a half hour using some basic shop tools.
Turn off the water at the shut off valve at the water main before attempting to remove the Stanadyne cartridge.
Loosen the hex screw on the underside of the shower handle with an Allen wrench, and remove the shower handle.
Unfasten the screws holding the shower handle cover plate to the wall with a screwdriver, and remove the plate from the wall.
Unscrew the handle adapter from the shaft by hand, and remove the adapter.
Loosen the faucet escutcheon by unscrewing it by hand from the Stanadyne cartridge, and take the washer and limit stop off the cartridge.
Take the retaining clip off the cartridge using a pair of pliers, and place the white plastic removal tool from the cartridge removal kit over the top of the cartridge stem.
Turn the cartridge using a pair of pliers until the stem works loose, and pull the Stanadyne cartridge out of the valve.