'91 Regal 270 Km.
I thought I was the only one who did this, but I met a guy recently when I was charging my system so I plied him with questions and he said, " I have 2 older cars and each year I pump in 1 Can of leakstop and then recharge them", and that is my story going back even to the late '80's with a newer model Skylark. I get all my stuff from Frosty Freeze on Ebay including a gauge. (He has some "How too's there too") 2 1/2 cans usually does it, and you know things are working when you first hear the compressor kick in early into the first can.
Also for those out there with Freon systems, I have used R-134a type fluid for 2 years and NO disaster has occured, altho' my supplier now has a Freon 12 substitute.
Here is a forum anyway for your perusal:
http://acsource.net/acforum/viewforum.php?
f=1&sid=4bb89225430c40fed750eb3810f4d1a1
Good luck