Fixing drip in Kohler faucet

I can't seem to stop a Kohler hot water bathroom sink faucet from dripping. Replacing the washer didn't help, and I replaced the valvet assembly. (I learned the names from an illustration on the Kohler site.) The replacement parts I used were from Danco. Numbers on the package were seat 56, handle adapter 25, position 79, # 15553B.
I guessed that the nylon "washer" attached to the bottom of the barrel isn't making perfect contact with the brass "body" (I don't know what to call it.) I tried using an extra brass washer on the plunger shaft (stem) so that the bonnet pushed the barrel with more force. That didn't help.
Any suggestions for a next step would be appreciated.
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