Your mention about holding vacuum for 24hrs is notable. I have long been a proponent of keeping R-12 in any system that was originally R-12, especially of you want to use the original condenser. Many balk about the cost, but in the scheme of things the cost of a full charge of R-12 isn't that much extra when the cost of refurbishing the entire AC system is considered. The main thing is to make sure the system is absolutely leak-tight. With modern barrier hose, neoprene (or HNBR) o-rings, and a tight compressor, an R-12 charge should last for years before a top-up is needed. All compressor shaft seals weep a bit, but as long as the system is run for about 5 minutes every couple weeks that leakage will be very, very small. I've probably mentioned all this before, but it's worth repeating: carbon seals are excellent, reliable seals, but they must never be allowed to dry out. They rely on an extremely fine film of oil between the carbon and the lapped seat to fill in any microscopic voids and prevent leakage. Over time, the pressure in the crankcase will slowly force the oil out from between the carbon seal and the lapped seat. If the seal is allowed co completely dry out, not only will the system leak, but on the next startup the dry seal will wipe itself out and likely never seal again. At that point the only fix is to replace the seal. The only way to avoid this is to maintain the oil film by running the compressor regularly.
All of the above is also the case for a system with POE or PAG oil and R-134a. The system must be run regularly to keep the seal lubed. Otherwise, same result. You will wipe out your seal and lose your charge. The only difference is that you won't cry as much when you lose a full charge of R-134a vs losing a full charge of R-12.
