Hi - when we pull up the lever on what I think is the mixer valve on the tub spigot, it doesn't seal all the way and the pressure to the shower head is low. About 1/2 of the water is still running from the spigot into the tub. How do I remove the tub spigot to clean away mineral buildup on the surfaces of the valve? Break loose the caulking and then does it unthread? Which way is the thread usually? Do you recommend any particular chemical to cut lime, etc? Thanks very much, Charlie