My regulator leaks a little and I have to make sure that I close the tank valve if I leave the welder unattended even for a few minutes. When the cylinder was full at 2200psi, I would open the tank valve, close it, and the needle would only take 5 minutes to hit zero (!!!), but once the cylinder pressure dropped to around 1500 psi, it stopped leaking altogether for some reason. Don't ask, I have no clue and simply attribute it to magic and Jesus!

I'm in need of another cylinder (emptied the first one welding!) so now I will see if it's actually the cylinder's seat or the ball on the regulator that is leaking when I get the replacement cylinder. I don't have another regulator or cylinder to verify which component is leaking, and I was planning to buy a decent regulator in the future anyways.

This may be obvious (but in case you didn't already know), you should "crack" your cylinder valve open for a split-second before connecting the regulator to the tank. This will eliminate any debris in the threads. I've even gone to the extreme of using a lint free cloth and some degreasing solvent to wipe both the ball and the brass seat clean (and film free) on my setup since I have had issues with it leaking. Check for gouges or signs of over tightening (grooves), plus check that your regulator ball is smooth and no plating is peeling (if plated).

Also (this may be obvious as well) but no oil, sealant, or any kind of tape should be used on the tank valve threads or regulator (seat threads).

Good luck with your "new" shoulder, that's quite the ordeal to go through! Get the neighbor kid to lift your replacement tank into place for you! I still can't believe it's $92.00 to exchange my 150 CF tank!