Aircon Pressure Switch Replacement

[Boy]

I need to replace this pressure switch on the aircon rad of my 2008 s Line.
I bought a genuine one from the dealer together with a new o ring.
When I unscrewed the old pressure switch the old o ring came off with it.
I have changed one of these before on my B6 but am a bit puzzled as to how to fit the o ring
before screwing the new switch onto the threaded part containing the Schrader valve.
It looks to me as if I should slide the o ring over the threads so that it sits at the base of the threads
and when the switch body reaches it, the o ring is compressed by the smooth tube below the female threaded
part of the switch.
Is that the correct approach?

[Gunnark100]

Yes, slide o ring first and sensor(5Nm, does not matter but) after!

[Boy]

Thanks Gunnar. That reassured me enough to go for a re-gas which successfully revived the system so I now have a cool car again.
I'm still puzzled as to why the gas was lost in changing the pressure sensor but assume that maybe it can happen in removing the old sensor.

[Gunnark100]

You probably lost gas/pressure already when old sensor was still attached, I had same puzzle...but faulty G395 leaks, valve should close very quick if removing(if not in slow motion) old one.

[Boy]

Good point. That is probably what happened.