Unlike many AC systems, your jetta does not has a sight glass window so you can't tell when it's full. Too much and too little will damage the system. I think there's a 20,000 fine for releasing freon into the air. You are supposed to use a vacuum machine to pull it out and then take it to a recycler.
All modern AC systems use the same type of freon, I forget what type it is, but you can get it at the parts store if you are feeling brave. I would just take it to a pro for this one. Plus, if it is leaking out, a pro will have a machine which can take out your freon, put in a flourescent dye and see where it's leaking from. I tried recharging an old car and it just leaked out a month later.
Also try checking the air filter for the cabin.
For regular maintenance of the system, run the AC once a month for a little while, even in the winter and it'll keep the AC system working forever.
May I throw a thought. The defroster is suposed to cycle the compresser to remove moisture from the air in the defrost cycle. I know this is the way the domestic cars work. Thanks eric davis AKA cadman1940
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