Ford Repair: Air conditioning question..., ring seals, stable characteristics

Question
Mr. O'Neal:
I've got an old '93 mustang, and the A/C freon is just about gone.  How complicated is it to convert an old R12 air conditioning system to an R134 system using one of those "retrofit" conversion kits?  If it's just a matter of chaging out fittings and then putting in the new "freon"  (or whatever it's called now), then I feel pretty good about doing that.  But would it harm the AC if there's still the least little bit of R12 freon still in there?  I don't want to have to take it to a shop and pay a disposal fee if I don't absolutely have to.
Any advice/suggestions greatly appreciated.
Warm regards,
David Gardner

Answer
David, the two different refrigerants (R12 vs R134) do not mix and have different properties... so NO, you can't just add the fittings and top it off!  If you did, you'd then have a system with a 'contaminated mixture' of refrigerant that is costly and difficult to dispose of later, and will not have stable characteristics for your cooling purpose.
You need to first find any leaks in the system - 'O' ring seals back then were not as good as the ones we have today, and were common for leaking at the condenser, evaporator, accumulator, etc. connections.  Reclaim the R12 that is still in the system - it's valuable stuff, so a shop that has the equipment will likely reclaim it for free.  Replace all the 'O' rings with the newer green colored seals from Motorcraft.  While the system is empty, drain the accumulator of all the oil in it, and add back 6oz. of Esther Oil.  Change the orifice tube to a blue one.  Then, clean the charge fittings and add the new type for R134.  Draw the system into a vacuum for at least 20 minutes, hold the vacuum and be sure there are no other leaks.  Finally, charge the system with R134 to about 80% of the amount the system originally held.
If all works well, you'll have cold A/C again using the cheaper and safer R134... but you may have to play with the cycling switch to allow the low pressure to drop more without turning off the compressor - adjust to about 26psi works well.
The reason for adding Esther Oil (not R12 mineral oil or R134 PAG oil) is that Esther will mix with either of the others.  The change in orifice size is needed due to the differing pressure characteristics of the different refrigerants.  
Hope this helps....
Clay