SPRINKLER TALK FORUM - You Got Questions, We've Got Answers

I have a well sprinkler system that has a valve box less than a foot away from the well head. The valve box contains only a five inch long vertical Schedule 40, capped, threaded, 1 1/2" PVC pipe. When I unscrewed the cap, it contained water a few inches below the cap. I stuck a small wire (about 6 inches long) into the pipe and didn't contact a bottom. Is this pipe to be used to attach a compressor to blow out the system for winter? It seems strange there wouldn't be a shutoff valve (to the well pump) in there if it was. I changed out the well pump a few months ago and I know the pump contains a built-in check valve. If this IS where I would attach a compressor, is there a danger of wrecking the check valve in the pump? Thanks for any help.