About a year ago I repaired a shower/tub cartridge exactly like
the one shown. I believe it was a Moen. I had to purchase a
cartridge puller ($10) to remove the old one. Home Depot sold
a repair kit (~$12) that included three O rings and two rubber
tabs installed on both sides for hot and cold flow. Chances are
one of the tabs is torn and not shutting off the hot or cold flow.
A problem with an O ring would cause the leak behind the
handle. Your leak is probably one of the tabs torn.
Make note of the original cartridge placement and put it back
in the same way. It is possible to install the cartridge upside
down. Home Depot did not have a replacement cartridge in
their inventory, so, be careful when removing the original one
as you'll have to reuse it with the new repair kit. If the rubber
tabs are torn inside, it may take a little effort to pull the original
cartridge back out, thus, I had to buy the cartridge puller.
Your local plumbing store may have the entire cartridge. I'm
sure he'll know what is needed.
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