To close out this thread for future reference, I’ll share that the leak was not coming from the o-ring. There was an extremely tiny pinhole in the hard line below the o-ring where the ferrule contacts the flare.
I put a vacuum on it and heard a faint hiss. I then pressurized the line with water. Even knowing where it is, I could barely see it.
You can temporarily put an o-ring below the flare to diagnose this. Also, keep in mind that if you have a leak at the pressure line going into the pump, tightening the ferrule can actually make the problem worse by cracking the line. It’s not a compression type fitting, it’s an o-ring that rides in a bore. The ferrule just needs to be tight enough to keep the line in place.
Now I just need to find a new pressure line at a decent price. MSRP is about $1k. Ack