Ugh! So this is an older model Moen 91111 two-handle faucet. The hot water tap had been dripping, so I swapped out the cartridges. Basic 1224 models with the little tab that fits into the notch in the collar...easy. Both collars and stems were pretty corroded from hard water. The cold went in easily but the hot was a bit of a problem. Used WD40 to clean it all up and cartridges and collars are seated and handles all in correct positions. The hot water side is still dripping same as before though. A drop every hour or so...not terrible just want zero drips. The hot water shut off valve isn’t working right, fyi. Even fully engaged it’s shutting the water off to a just a few drops...the cold shut off is fully engaging. I tightened the sealing nut on the shut off but nothing...maybe I’m not strong enough to tighten it?(im a woman btw) So I’m having the plumber come fix the shut off, but wanted any advice on the hot water drip. Is there some other seal somewhere that could be bad? All I can think is the aerator sealing washer but I’m stumped for sure. Any advice besides leave that for the plumber too?
There is probably some hard water deposits inside the valve where the cartridge O rings and seal make contact. The slightest deposit will break the seal and you'll have a drip. Get some fine emery paper and gently clean out the valve making sure it's totally smooth inside. Then get some plumber's grease, coat the cartridge and reinstall. Hopefully you didn't damage the O rings inserting the cartridge the first time.
This will happen sometime when the valve body has corrosion in it preventing the O rings from seating properly. Remove the hot water cartridge and check the cartridge seat. You may need to use a small flashlight or try to feel down in the valve body with a finger. My guess it will feel rough. Clean the valve body out with a decalcifying agent and a wire brush being careful not to damage the surface. You can also use some 0000 gauge steel wool. Once it's smooth reinstall the cartridge and you should be good to go.