Can anyone tell me how the o-ring works for this switch? Does it sit into the blue area, or does it go on the stem? I bought a leak detector and it's detecting a leak around this switch.

 

It goes on the valvestem. Be sure to lube it up well with PAG oil, or else it can get pinched.

 

Right: there's a groove on the valve stem, that it goes into. Which is possibly why it leaks, with it the way it is now.

 

it had a black o-ring, but it was pretty dried out looking. I bought a multi-pack of the green a/c o-rings from advance auto, but they don't seem right for some of these connections, either too loose, or so tight that they seem stretched.