make sure there is enough r12 in the system. Thats the easiest way. Look on the top of the reciever dryer at the sight glass and see it there are bubbles. A fair amount of bubbles means the system is low. Also make sure your condensor is free of bugs and mud, and that your fan clutch is blowing enough air.
As for the 134a conversion, they sell kits to that, with o-rings, oil and port adapters. Yes, r12 needs to be recovered before you can convert. And at the price r12 sells for, it would be wasting money to just purge it to air whether or not it was harmfull to the ozone.
If you want to do the conversion, I would say go to a shop and have them recover it for you. Maybe work out some deal where they get the refrigerant and you get the labor for free. I mean, I just bought a half pound of r12 for $40. Then go home and install the conversion kit. You should be able to charge it yourself as well.
