i went to Target today and saw a refil kit for 134a refrigerant.
it also comes with a gauge to check the levels.

I was amazed when i saw this. And its only $30!!!

Has anyone tried it?

I uses the low pressure port in the system for charging.

When i would unscrew the cap, what should i be expecting?

 

All you should have to do is plug the hose on the canister into the low pressure valve and press the button on the can... These kits usually contain refrigerant, compressor oil, and some other additives to refresh the whole AC system. Never used one (sold them)... Personally, I wouldn't mess around with something as complex as the A/C unless you already have a problem. Overfilling refrigerant will destroy your compressor... Be careful!