Regal 250 EXT Air Pump Leaking

I'm setting up my Regal 250 EXT skimmer and for the life of me I can't get the inlet of the air pump to stop leaking (part J on the exploded diagram). I did add the O-ring shown on the diagram. It's not leaking from the thread connection to pump, it's leaking just upstream of that - where the "union" piece connects to the threaded adapter. I tried tightening to no avail. Any ideas?