Classic/Antique Car Repair: R-12 AC, lincoln town car, r134

Question
I have a 1989 Lincoln Town Car with 74,000 miles that is in good condition, except that the freon in the R-12 AC system has leaked down over the last year or so such that the system no long cools well.  The o-rings were replaced 5-6 years ago and the system was recharged and worked correctly until last summer, when I noticed a decrease in efficiency.  I considering recharging again with dye added to indicate the leak.  I consider this more reasonable that converting first and then checking for a leaks and then replacing additional parts plus the conversion.  What do you think of this plan?

Answer
The only problem is...you will have to recharge the AC before you add the dye, so that will waste R-12 looking for the problem. If you do not use the AC on a semi-regular basis, the o-rings can have dry out over time. However, sometimes the problem is the condenser hose leaking or the condenser itself, but dye is still the only way to be sure. Overall, it maybe cheaper and less problems if you convert first and add the dye. You avoid the cost of R-12 and the R134 may fix the problem. Either way, if you have a leak, it will have to be fixed and recharged and R134 is cheaper than R12. That is the only benefit for converting first.