Gotta be careful with picking up non-sealed cylinders... Almost picked up a partial Freon 502. I know it wasn't nearing anything close to empty, because the liquid inside could be felt. Checked pressure and it was only standing at 30 psi at 75F. No clue what the hail was in it.
Glad I checked it before I got it. Almost got myself a costly waste to dispose of. There are/were even kits to adapt them to air storage tanks.
Even many side can taps and top taps for small (12oz) cans have a check valve.
Do any of the large (20-50lbs ) disposables come with a check valve to prevent fraud and unsafe use? When did they start putting them in and is it mandatory these days?