take it to a air con shop - they connect both sides low and high - then they evacuate to a vacuum first - if it doesn't hold they track down a major leak - if it holds vaccum , then they refill with the correct amount of new freon by weight.
Topping off a air con system from one side with a can of freon is cheap - but often will not do the job.
The aircon service with the machine/vaccum/etc this time of year should be under $80 at a good air con shop.
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