It is leaking from the Schroeder insert, which is where all of them have leaked. I owned the car for about 9 years before it became a problem. I don't know if it's leaking around the insert (i.e., threads) or up the middle of the insert itself.

I do have the cap on.

I will try replacing the outside o-ring. You're right, it has probably seen its share of wear by now.

As an extra measure, I have also put an o-ring inside the cap, so there would essentially be two o-rings for the cap. Refrigerant still seems to escape, albeit gradually. I have checked for leaks elsewhere and had the system professionally checked. It's not leaking from anywhere else. I've also had two Schroeder valve inserts professionally installed, with the same result. The first one lasted two years. The other about a year. And I have only bought valves from the dealer hoping to avoid that problem.