In all my years of pump/well service I never had to replace an O ring in a pitless. I would clean our the O ring grove the best I could, dry it and install the O ring dry into the groove. Then lubricate the face of the O ring and reinstall the slide in portion of the pitless and his should do it.
Always keep in mind everyone is an expert at a distance. . . you are there and only you really know what's happening. A friend of mine says that a "Consultant" is a mechanic from out of town and he's pretty much right!