Hello, it sounds like a leak, a slow one but not real slow, many freon charge kits come with leak sealer in it , some do not, I understand they checked for leaks but sometimes that is difficult. One needs to add a dye to the system, run car till ac quits again, then look for leaks , looking at the UV dye, It could be a line connection or compressor front seal or, the one you can not see? the evaporator behind the dash, In order to confirm leak is at the evaporator? on need to rule out all possible leaks under the hood, its a process of elimination, To save money? you can buy a R134 recharge kit, add the freon that has leak stop in it, then add a can of the dye, Instructions on both will be good and you should not have any trouble doing this on your own, Save lots of money. You hook the can to the low pressure port, pull trigger till guage shows in the full charge range. Then add the can of dye, All can be bought at parts stores. I have included a link showing where the low pressure service port is as well as picture of the can of dye, well one brand.
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