They are disassembled for a thorough cleaning, and then correctly reassembled.
The O-Ring is at the very back of the faucet, where it screws onto the tower. Those do not leak, never have.
The issue is they simply won't stop flowing. And/or dripping, from the faucet. The forward seal feature is not working at all.
A very small O-Ring is internally located, at the forward part of the faucet.
As of now, my dozen or so Perlick faucets (3 generations) are put away in a box, with my Tap-Rite faucets on the front line.
Those of you who warmly gush over your Perlicks, good for you!
What is the best source for replacement parts / front seat O-Rings?