There is no O ring, it's a plastic screw on cap, I ended up looking for and finding a non hardening Teflon sealant I brushed on. Last nght it didn't stop the leak once it pressurized....I still had a slow drip. Didnt drip when punp wasnt on. This afternoon it stopped leaking though. Maybe it just needed some time to set or something. So as of now, it seems okay. I'm going to bail it next time, seems like a pita but would eliminate any future leak issues. Thanks guys!