Keg leaks when not connected

I have one 5-gallon ball lock corny keg that seems to leak whenever it is not connected to the CO2 supply. If I disconnect, then by the next morning there is no longer pressure (no ‘pffft’ sound when I open the relief valve). I am pretty sure it does not leak when connected because I had it hooked up for a long long time and I think I’d notice the CO2 tank running out if it was leaking. I have replaced the gas poppet (actually, I think I replaced it twice) and it still leaks.

So, is this a mystery or what? Any ideas for what I can do here?

I was gonna say the gas poppet based on it only leaking while disconnected. Are you properly sealing the lid? I’ve never used it, but heard of others using keg lube to stop small leaks.

Kegs can leak through the lid, poppet, or the post seals. If it isn’t gobbling up gas when connected it probably isn’t leaking through the lid or post seals. You would probably hear that because the tend to be faster leaks. The most likely cause is the poppet. You could test this by putting a bit more pressure in the keg, like 30 psi, removing the gas line, and spraying with soapy water or star san solution to see if there is bubbling or hissing. Often slight misalignment of the poppet will lead to a slow leak. Sometimes just pushing it in or sideways will cause it to re-seal properly. If that doesn’t work, or is only a temporary solution, I’ve had success taking apart the post and using lube on the rubber seal part of the poppet. Alternatively, there may be some piece of junk in there interfering with the seal. That is much more likely to be the cause on the liquid out side where small poppet leaks result in beer slowing filling the bottom of your fridge or freezer. I’d say you have the preferable case!

Are you still force-carbing the keg? If so, the pressure in the headspace will continue to drop until it is fully carbonated if not always connected to the regulator. Once it drop below a certain threshold, the seal might be lost and then the beer will go completely flat. To force-carb a keg with an iffy seal, you may need to keep it connected the whole time OR crank it way up and hope you went high enough to keep the seal (but not too high to over-carbonate)