That procedure of filling the system with air seems bogus and very detrimental to the system. First off, you cannot find the leak using air. Air will not register on a leak detector. Real shops use a mixture of nitrogen with 5% R12 freon so the detector has something to pick up. They pressurize the system up to about 300 psi and then go sniffing to find the leak. Second, putting air into the system will carry moisture in with it. You don't want any moisture in the system. Third, running the system with air in it is not the same as freon because the air will change phases like freon does. And as a result the high and low pressures will not be like freon. It will not carry the lubricating like freon etc. Thats not doing your compressor any good at all. Also a very small leak will be hard to detect any loss of pressure over a short period of time.

I have had to chase several leaks in my cars and they have all been O ring related in the hoses. Usually in the high side connections at the compressor or hoses on the high side.

Sounds like you need to take your car to a shop that knows what they are doing.

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I got too many cars!! Insurance eats me alive. Dave

78 Corvette Stingray - 3k
82 242 Turbo Volvo - Manual - 270k
86 300e 5 speed manual - 210k
87 420sel - 240k
89 560sl - 78k
91 420sel - 205k
91 560sel - 85k
94 GMC Suburban - 90k
97 Harley Davidson Heritage Softail - 25k
00 GMC Silverado 1 ton 30k