After having the shop that repaired my R12 AC system last year mildly to moderately under-fill it (before it was ice-cold; now it cuts the humidity ok but only gets cool only after 10-20 minutes of highway driving) I got the necessary certification and am planning to top the system off to get it up to the level it should have been filled to. In order to avoid doing something stupid and releasing a bunch into the atmosphere (and wasting money), I want to make sure I'm prepared with the right tools before I start.
It looks like all the small R12 cans you can get are sealed and require a tap (as opposed to DIY R134a cans with valves on them). I've seen top- and side-tap devices advertised for this; some examples:
How do they work? Do you latch the device onto the can first then twist the tap to puncture the can, and loosen it again to allow it to flow? How do you seal it to prevent leftover coolant from slowly escaping? Do you need a secondary shutoff valve of some sort attached?
Aside from this, are there other tools I need or should have on hand? I do have a set of hoses/gauges already, but in the past (R134a systems) I've just gone by feel of the air blowing out to judge when it's suitably full.