"in a previous life" I spent nearly 15 years in HVAC / R service, and acquired a number of empty refrigerant jugs. I used to use them for several things
Small one..........pressure brake bleeder
30 "pounder" pressurized 90 wt bottle. Brazed a pipe nipple into the top for filler, brazed in an air fitting, and brazed a tube down to the bottom for a pickup tube.
I used to use them for air bottles until people started worrying about them rusting out......a legitimate concern!!
Cut the tops off and they make a "dip tank" for carbs and other small parts
Unfortunately, I "don't get to get" them anymore LOL