Well, anything is possible.

Put it this way. If your comp was leaking anyhow, then you'd need to do a conversion to R134 sooner or later (or spend the cash on 12). That is, when you changed the compressor. Or compressor seal.

But then you should change the accumulator tank too, since it's been opened up to the air.

What I'm saying is, if you have another problem, here's what to do: Flush the system, change the accumulator and maybe the compressor, fix the leak, and go on your way with 134. The 91 system was designed for 134 to begin with, with the cooling fan and a different condenser.

/// I looked several places and the authority is SAE J1661, on the AC conversion. I won't have time to go look it up for several days/weeks.

What the manufacturer recommends is also the final word. There are seal materials effected by R134a and other seal materials are not. One of the recommeded oils used is chemically reactive with some seal materials and incompatible with mineral oil used with R12.

Find out what the manufacturer for your system says do and do that.

What is probably happening is that the kits used eliminate any possible problem regardless of your origional seal material.

I believe it is just a mater of time. The lubricants already in your system aren't surposed to be compatible with the R134. The pump will eventually die from being poorly lubricated I believe.

The system needs to have the oil flushed and replaced to use 134A, the expansion vale may also be replaced.

Use Freeze12 instead.

You are asking for trouble unless your mechanic swapped out the mineral oil used for R12 for ester oil that the r134a needs. That is the absolute minimum for a refrigerant swap-- some argue you should also change the receiver/dryer and possible the orifice valve and o-rings.

Good luck!