wal-mart, autozone sells a 10 or 12 oz can of freon that comes with a leak sealer for about $6, I believe made by Interdynamics Corp. This comes with instructions and you can't go wrong when there is only 1 port.

I wouldn't use a 19 oz can, is too much, better use 2 10 oz cans. The total ac system holds 1 lb 7 oz for a total of 23 oz. Thus when the system is low a 10oz can should bring the system back to fullif the leak is minor and sealed by the sealer. If you have a major leak then your system is empty and a 19 oz won't cure your problem since the freon will leak again.
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'88 240, '92 745, '98 v70 John, Tampa Bay