Full charge after only half the freon?

[JohnHere]

Conversions from R-12 to R-134a can be very tricky. You could start by adding refrigerant in small amounts (an ounce or two) while watching the pressures and vent temp. An additional person would be very helpful with this process. If the pressures start going up, along with the vent temp, you can be pretty sure that you have reached or slightly exceeded the optimum refrigerant level, and to stop there. You could even reduce the refrigerant at this point a little bit, maybe by an ounce, and call it a day.

Most occasional A/C techs can't afford the thousands of dollars an RRR machine would cost. But if you plan to continue working on MVAC, and assuming you don't have one, I suggest investing in a high-quality refrigerant scale, which will allow you to charge precise amounts of pure refrigerant from a 30-pound canister...much easier than trying to accurately charge from multiple 12-ounce cans. Speaking of the small cans, some of them might also contain sealers, which you definitely don't want to introduce into any MVAC system.