Before I had the proper requirement I played around with cans. First of all the cans with the gauge cost too much, they have some bogus additives and alot have some ?? oil or worse LEAK SEALER. Small quantities but still.. You always wind up over charging with the can. If the system is leaking, it has moisture and some air in it, no questions asked. When I first recovered my can charged system the vacuum pump oil contaminated BAD with moisture
If you care to fix it right Look for a leak with a UV light or a sniffer The factory AC fill has a UV dye in it already. At least it did on my car. About 4 points were lit up like Christmas. Case seals on the compressor, shaft seal on the compressor, high and low valves. Its good to change BOTH service valves. The low side is just a screw in schrader like a tire, but its for AC. Don't use a tire one. The High side requires a special 8 sided socket. All valves leak with age. People put the can up to the low side valve, the effen valve will just leak and will never seal again from age. . I had it happen many times on many vehicles.
Recover. Repair leak, Vacuum, note that it holds a vacuum. Charge Charge it with a scale or a fancy machine... get a good R-134a cylinder when you can, probably it will be banned soon Spend $100- 130 , 30lbs, it will last forever,.. Anyone can buy it license or not. Some stores are A holes and I have to prove myself that its not r-12 . And I do have a card
I would also recommend maybe throwing a new orifice valve on it. PITA, need to remove the battery to get at it.... The drier, accumulator ahh. I would leave it . Think the new Chinese ones are better than the OEM one? Ill try to run the OEM one as long as possible.. Cleaner the orifice valve, better AC performance.
I don't buy a system needed a fill from time and leakage. That's 1970s talk. I have a 11 year old 130k vehicle here that has been running the AC every day for hours and hours since it was new. It does not need a top off. That whole "the compressor shat seal leaks oil and refrigerant to lubricate...." is bs If it leaks, you have a leak.
Snoop around and you will find a leak on that "It only needed a top off" system
if you search around you can probably find some shop who will Service the AC for 60 or less . The people who want 80-90 can screw off.