Check valve? No idea. I bought the regulator and tank from Central Welding in Redmond (up near Target and Home Depot), so it's whatever comes standard with what they sell. It's got the 0-2000 psi high-pressure gauge, 0-60 psi low-pressure gauge, a relief valve, and a 5/16" barb outlet with a shutoff valve. Maybe there's a check valve built into the shutoff, but I don't know and I don't know how to check. Incidentally, though, there's a "60" stamped on the relief valve. Just out of curiosity, I cranked up the regulator until the low-pressure gauge pegged (so it was definitely above 60 psi) and the relief didn't do anything. Faulty valve, or is this the wrong way to test it?
At any rate, I dumped the 5 psi that it was showing yesterday and 24 hours later it's still at zero, so there must have been something residual in the system somewhere. I've got the tubing and such I need for the carbonator on order, so hopefully they'll get here within the week and I can start playing around with it. I've got lots of ideas for stuff—mostly non-alcoholic—that would probably be better if it was fizzy.