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Make you a strong dish washing soap solution, just like to find the leak in a tire, and check it all, all coils, and connections. Now an auto, with a sight glass is a bit different than a home system. My question is.. is it a heat pump, if so the only real way to charge it accurately is to evacuate it, and put the calculated from the mfg.s tables charge in it. Not too many folks do that. Most folks that do it regularly get a feel, according to the outside air temp, the inside air temp, and what the pressures are running and can get close. I think you can learn to do it yourself with a little help. If I can get the inside temp, about 20 degrees lower than the outside temp, and get the low pressure manifold gauge running around 40- 44 degrees on the R22 scale on the gauge (evaporation temp) and the outside high pressure running around 225 - 250 lbs of pressure, it is usually good to go. There are other things to look at as well, the humidity, as well as the amount of condensate running out the drain pipe. Charging it and guessing it takes some experience, but there is only one way to get that, and that is to do it. The ONLY way to do it accurately is to evacuate it, and measure the charge by the mfg.s recommendations. I hope that made some sense... If not, Oh, Well..
If you get it overcharged, and get liquid refrigerant coming all the way back to the compressor before it evaporates it will wash the oil off the compressor bearings, and shorten the life of the compressor quite a bit.
If you get it undercharged, you are paying for more electricity than you need to.
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