A couple of years ago, I was given a dozen propane canisters of the type we use in our barbeque. After using one of these canisters for the first time earlier this year, I unscrewed it from the barbeque control valve but immediately realized that it was leaking propane from the screw connection at the top of the canister. Once home, that canister was replaced with another one from this gift and it also leaked after using it the first time and unscrewing it from the barbeque.
I now think that the problem is due to the age of the canisters. I don’t know when they were originally purchased but I suspect that they are at least ten years old. The canisters incorporate a built-in valve which opens when it is screwed into the control valve. Due to age, this valve doesn't close up with the canister is unscrewed.
Just a heads up…
(The second instance this problem was near the beginning of a week-long trip and we had planned to barbeque several nights on the trip which crossed over the southern Sierras, a long way from any stores. By removing the control valve from the barbeque itself and screwing it back onto the canister, I was able to stop the canister from leaking. However, I didn’t want to store the canister inside the van since I wasn't convinced the solution wouldn't leak. My solution was to put the canister and control valve in a zip-lock bag (to keep dust out of the valve) and then put the “package” in our Trasharoo mounted on the back of the van. This let me continue to use it during the rest of our trip without worrying about propane in the van itself.)
'01 Ford EB50p Quigley 4WD