leaking around shower valve cartridge

Five years ago I replaced the Moen 1200 single-faucet cartridge in my shower. Now it drips from the knob. It seems to be leaking around the cartridge.
I found the plastic adapter and, after tapping the pipe, got the cartridge to turn. Drawing it out will be another matter. I don't recall that the one I replaced was hard to pull out, once I turned it.
How should I get this one out? Is there an anti-corrosive/sealer/lube I should use when installing a cartridge? Should I not remove it until I have a new cartridge on hand?
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I've found the easiest way to pull a Moen cartridge is by the use of a slide hammer/dent puller. Just fit it with the right screw to fit the cartridge and give it a couple of smart whacks. I have found it is usually hard water residue that has built up around the cartridge that holds it in place. And yes, have the replacement in hand before pulling the old one.
Charlie
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