The EPA goal of course is to get rid of R12 entirely. My interepetation of the law is that it is not legal to convert back to R12 once a system has been converted to 134.

This makes a good arguement for not switching to 134.

You may find it dificult to find a shop to change it back for you. In fact I found it impossible to find anybody that would restore it to R12.

To do it yourself, you would have to go into a shop, have the system evacuated, replace the dryer, replace the fittings, flush the system, then have it evacuated and charged with R12. Flush the compressor with the oil you are going to use. You can buy R12 on eBay. Certification is a joke. Take a test online and pay a fee. the important part is that you have no excuse for doing something stupid if you get caught. You can't say you didn't know any better.

This is of course, not only a lot of hassle, but also illegal, so don't do it.

A dryer designed for R12 is not compatable with 134. I doubt you could find one anyway. Anything made in recent years would be 134 compatable, and work fine with R12. If your system is R12 and the parts guy says all they can get is a dryer for 134, smile and say "Thank you that will be fine." "O" rings also, You could get cheaper ones, but go ahead and use the green things for R12.

As to the number of hours that the system has to hold a vacuum, it is on the certification test. The system must hold 25 inches for at least 2 hours before recharging or something like that. I think it was 25, the 2 hours part I am sure of. Overnight is better.

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69 220D not running
79 240D parts car
80 240D Beater runs everyday
81 300TD temp disabled cooling problem
82 300SD sweet