ball valve o-ring?

[sclose]

I am trying to find the source of an air leak. When I shut off the ball valve to the main drain, the amount of bubbles in the filter basket goes way down, so I think the leak is in that line somewhere. I took it apart, and I see a groove in the inlet pipe for the valve, should it have an o-ring? Photo attached.

 

[HermanTX]

That groove appears to hold an o-ring. Was there any pieces left in it? did this leak just occur? It seems strange that the oring is missing and if it is, then the leak would have happened when the valve was installed. Has this always been an issue?

It may be helpful to show the mating piece and if you can determine the manufacturer name.