You've performed most of the troubleshooting, but the service person does it again, anyway...
Have you ever had a leak in your GCMS and you couldn't tell where it was coming from? You tightened, retightened, and double-checked everything? Then you call the service tech and he tells you to redo everything you did.
I know this well because I just had a call like this. Please don't feel like we don't trust you. It's just how our internal troubleshooting process works. It's a "checking all the little things" practice that works surprisingly well for most of us. Most well-trained Service people will start at the simplest troubleshooting steps and, with a little bit of intuition that comes with experience, will jump onto a subtle detail that may not have been caught before. These little details can lead down an entirely different route than was being followed before. Also, two sets of eyes are always better than one!
The little tidbit of knowledge I'll hand off: next time this happens, grab a can of good old-fashioned canned air. Yes, the one you use to clean off your keyboard. It has a chemical added (a bitterant) listed on the back with the CAS number. Use NIST and search for the CAS number.
Set your MS to scan +/- 5m/z of the base ion of that CAS. Once you're actively scanning, spray a small amount around common leak points and o-rings. Once you see that ion go crazy, you've found your leak.