When you unscrew the old nipple assembly, check the inside of the air tube and make sure there are no burrs that will tear up the O-Ring. Add a bit of silicone grease to the O-Ring before screwing the assembly in. Should help protect the O-Ring and keep a good seal. Doesn't take much silicone lube, just a really light smear. Ernest Rowe of FX fame uses Super Lube. I'm sure others will work fine. Just nothing that will combust under high pressure.
I spent many hours on my Marauder trying to get O-Rings without tearing them to shreds, finally had to sand down all the burrs on tine inside of the tube with 1000 grit then 2000 grit sand paper so it's glass smooth now. Really helped. Good luck to you. Working on guns is always fun!