Commercial beer keg couplers always come with a check valve. Often they have two. Why? Because without a check valve beer can go up the CO2 line into the regulator. The most common check valve looks like this: It goes on the edge of the coupler where the nut on the end of the gas line is attached. Should you have a full keg, forget to turn on your CO2 first, and not have a check valve, beer will back up through your gas line—often into the regulator itself. If you have a full keg and a totally empty CO2—the beer can even go into the CO2 cylinder.
In all cases you’ll need to remove the gas line, run water though it and maybe sanitizer. You can reuse it if you dry it out. Too much work. If the beer gets into the regulator, your best bet is to just hook the regulator up to CO2 and let it run for a couple of minutes. If it backs up into the cylinder you’ll need to purge the cylinder. All of this takes time.
The other kind of commercial check valve looks like this: It is held in place by a retainer that looks like this: This ball check valve is use from coming out of the keg and is inside the probe itself of the coupler. One fairly common problem with that ball check valve is that it can get stuck at the bottom of the probe. When that happens, it will stop all beer flow out of the keg. Your CO2 will be on, the keg will be full or full enough, and coupler will be attached, yet nothing flows. If that ball valve is stuck—easy fix. Just disconnect it from the keg and use a nail or screwdriver to push the ball back up the probe. That usually means you need to clean your beer lines as well.
What about ball and pin lock valves on homebrew kegs? Same problem, except homebrew couplers don’t come with check valves. If you have a pressurized keg, it’s full, and the CO2 isn’t on the beer WILL back up through your gas line. You’ll have to go through the same cleaning regimen that people with commercial kegs have to do.
This is especially true of roasters of cold brew coffee kegs that have a nitrogenating stone. In that case, the keg doesn’t even have to be full, even if there is only a little coffee in the keg, it will back up through the gas line.
We can’t recommend enough that you should have a check valve to prevent that from happening. The check valves look like this: It attaches to the grey “in” or gas coupler, either ball or pin lock. There is a second kind of check valve which replaces your existing coupler. It looks like this: In our experience, it isn’t as effective at the dedicated one above. We have 4 cold brewed coffees on tap and have had the one piece check valve fail on us half the time.
There are two other kinds of check valves that can be used, but they don’t prevent beer or coffee from backing up though gas lines. Almost all CO2 and Nitrogen regulators have a check valve like this: It is attached to the bottom of the regulator where the gas line attaches. It prevents liquid from entering the regulator. The other type of check valve is an inline one: or Beer or coffee won’t back up behind any of these, but can still enter the gas line.