Jason,
This is not really necessary.
I have been hearing about a new oil that you put into your system, that
causes the ester oil for 134, and the R-12 oil to be totally compatible.
So then you can install 134, wit out changing O-rings, dryer, etc.
And this stuff is cheap, and better yet it is sold at Wa-Mart !!
This does work---have a buddy in KY. that used it in his wifes Jeep a
year ago, and it is still blowing cold.
The advantage of using the 134 is that it is available anywhere, cheap.
I am a rock head, sometimes, and I have been very reluctant to use the
134, but the price for the R-12, or it's compatible replacements are too
expensive, and not readily available.
I have been using 409-A since the new EPA laws arrived, and it does work
excellent, but it is not available at your local store, and it costs
$225.00 per 30 lbs. bottle.
No small amounts can be had.

Good Luck,

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Terry
74 Westy (Clementine)
85 GL