So the guy that did the first repair didn't check for vacuum, right?
If they have to keep refilling the refrigerant it's leaking out somewhere. While the guy were doing mine I asked what they would do if it leaked, they said they had refrigerant they could put in that has a UV dye in it, then with a UV lamp and special goggles they can trace the tubing and see where the dye is coming out.
I do some work on our truck a/cs and I installed my own mini split unit in my house. I pressure them up with nitrogen first and leave it for a few hours and if it does leak down you can buy leak detection fluid, which is basically soap but is suppose to be safe for copper lines, then after that test passes I vacuum and let that sit for awhile.
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