Actually I have a deadly sensitive leak detector that cost quite a bit but soap is also good. Problem is I have found some miserable leaks the hard way on the newer aluminum York and cast iron Tecumseh compressors. The worst was a York that was brand new remanufactured and this system had a mysterious leak that only happened when running. It was a hairline crack in the mounting bolt area and the only way I found it was a very high pressure test. On agriculture or off road heavy duty equipment systems you will typically find text book operating pressures but automotive with thermostatic viscous fans that don't cut in until things are roasting hot can give pressures sky high on the automotive systems, enough to make ones hair stand up!! After seeing sky high pressures in some automotive systems prompted me to really raise test pressures and this was the only way the leak showed up on the York! Under cooler temps and lower pressure things were fine. On hot days and with chaff and dust on the condenser raising the pressure, this is when the system would leak, when it was most needed. Over 300 PSI was needed to expose the leak and normally the off road/Ag equipment has much better cooling capabilities than automotive and pressures this high are seldom seen unless safety switches are bypassed and the condenser is plugged. As for the 6.5 pounds, typically on the R12 systems the rule of thumb was charge until the sight glass clears and add another 1/2-1 pound more. Keep in mind that is liquid in the sight glass at this point and the extra half to one pound is stored in the Reciever Drier and the drier on our Mark II's are quite simply MASSIVE. The amount can be reduced if a much smaller universal/aftermarket drier/accumulator is subbed for the original. As it has been mentioned in the past with R134 the amount used is only 70-80 % of the original amount of R12. I remember when it first came out in the early 90's and all of the extreme horror stories of what was required to convert including replacing the R12 components with larger such as the condenser in front of the radiator or the evaporator in the cab. In reality being a new product we assumed you charged until the site glass was clear. Totally wrong and in reality by the time the glass was clear you were grossly overcharged! It eventually came to the point of charging to maximum efficiency and no more and I had to literally use a digital thermometer in the discharge with the engine at high idle roaring away and very slowly charging watching the temperature drop. When it quit dropping I shut the gas off and if one was distracted or day dreaming you would actually see the temperature start to rise indicating overcharge state. The lowest temperature achieved by this method was in reality the highest possible efficiency obtained and in some cases I found 70% was enough instead of the recommended 80-85%. When properly charged you will see a few bubbles swirling around in the site glass or even a milky appearance which is normal. Other horror stories were the entire flushing of the system because it was thought that you had to remove ALL of the R12 mineral oil because the R134 oil was not compatible in any way shape or form but later Polyolester oil came out and is compatible with small amounts of the R12 oil and compatible with ALL PAG type R134 oils so this is all I kept on hand and if memory serves correct it could even be used in an R12 system. Compressors on the shelf could be filled with Poly oil for both systems. It got to the point that a 30 pound jug of R12 was $1500 so it was cheaper to convert to R12 but in reality that was $50 per pound not $80. But then $50 was worth a lot more back then than $80 today LOL
Another point about R134 is it leaks out of old style hoses a bit more so the story goes due to a smaller molecular characteristic and would be like using Nitrogen in tires instead of compressed air. Barrier hoses are now used and I think they have cotton braided barier hose now that can be used instead of the smooth rubber hose.
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