I've been doing a/c repair for over 20 years now and I can tell you that 80% of the repairmen shouldn't be allowed to work an a/c. An overcharge is very common and your right, it is impossible to prove fault. Finding leaks takes time and good tools. Most guys don't have the patiance, and will not spend the money for the right tools. Putting dye in a system to find leaks is not a good idea,and can cause more problems.Before an a/c system is recharged it should be evacuated to at least 500 microns (I prefer 400), and able to hold that for at least 10-15 minutes,that will show system is tight and leak free. Refrigerant should be weighed in as per manufactures specs then checked by pressures and tempatures. Good luck finding a good repairman they are few and far between.If you were anywhere near western Pa I'd be glad to help, but since you use celsius I'm guessing your not from around here.
