You probably don't have any R12 left in the system or very little, just bleed off what's in there and recharge it with 134A. With the engine running and the AC on Max and Hi fan, charge it until the low side line coming out the firewall starts to frost, then bleed off enough gas so the frost disappears, should be good to go. Buy three 12 Oz. cans. Get one can with oil in it.