Where it may be illegal is that you (or someone) have converted the system to R134A, and therefore you can no longer claim to be grandfathered in for using R12. You are not repairing an R12 system. You are converting an R134A system to R12. That, in fact, may (note the keyword may, which indicates that you should ask someone who is better qualified in law than a bunch of car guys) be illegal.
I would recommend an A/C specific flush solution instead of some generic chemical such as mineral spirits.
Minerals spirits can (lesson learned years ago) soften and swell rubber seals and in turn cause them to leak like a sieve.
No idea why the problems with 134 exist in this case. I’ve had a few conversions that were a little balky but usually remedied with a bit of mild tweaking.
R134 was supposed to save the planet and when first introduced I told my wife that give them a year and the doom and gloom will start about that product. Sure enough, a year later a study in the U.K. stated that 134 causes testicular cancer in men.
No doubt the same thing will occur with whatever is presented next.
shadowfax.
You don’t know the EPA regulations when it comes to mobile AC and refrigerants. Why you do think some automotive AC tech’s have access to R12? Because we service older/collectable vehicles that won’t tolerate R134a.
As long as you’re licensed, you can convert back and forth all you want. As long as it’s within the EPA regulations.
Tester
“With the leaks now isolated and repaired, I’m certain the systems are 100%.”
That may be true for the moment but R12 is just too expensive to take a chance with. Sooner or later your 25 year old AC system is going to bite the dust. R134a will just make life simpler. Good luck.