Well, the official answer is that you must not mix R-12 and R-134a, therefore on the conversion you have to completely empty the system, which can be done using special equipment only.
I still wonder why, and what are the bad consequences of mixing the 2 gases.
I know quite a few people, who are simply filling up their 'old' system with 134a, and their a/c works fine.
If someone knows the answer please share...
Tiv
