I am putting in a new compressor and dryer,i had 134 in,i am going to switch back to R12 I know I need to use mineral oil..is there anything else I need to do?

Why are you switching back?

You are going to run into a serious problem if you do.....you are never going to get rid of all that pag oil in the system....it will be trapped in the evaporator and condenser, lines , etc.....

On conversions, you can mix a little mineral oil with pag, but the opposite is not true.....

If your heart is set on R12, and you have access to it, make sure you get an R12 compressor,and you must do a complete "flush" of the system, to get rid of the pag oil....

How do I get it out of there I have a 30 pound can of 12, pulling vacuum won't get rid of all that oil ?

I used ester oil when I had the R12 and I just used fittings to put the 134 in..I did not use any pag oil...unless its in the R134 cans..I was told that you have to reset the POA valve for 134 also..

Congrats on that 70 Chevelle classic, from an owner of a 71 Chevelle SS. I assume you're wanting to return to the R-12 system to make your car as original as possible and my hats off to you for that. On my part my Chevelle did not come with factory a/c and to be able to drive it year around after moving from the NW to SC I installed a Classic Industries aftermarket a/c system, my Chevelle original? no, but I can drive it when I want and that'* what is important to this original owner old man. Sorry I can't give you any advice on a R-12 system, just wanted to share my story with you. Good Luck.

I have it going with R134,4 cans I put in of the 12 oz,still got bubbles in site glass..i will drive it a little and see what happens,it goes to 60 and little lower on vent temp guage..I was told you cant go back to R12 after 134 unless you replace everything..oh well.

The sight glass isn't an effective indicator with 134a, only works with R-12.
Yes, I think you should use R-12, the system was designed for it and it is more
effecient than 134a. To go back to R-12, you have to flush everything but the
compressor, replace the accumulator,run mineral oil thru the compressor to
flush the other oil and recharge to spec with R-12 and proper amt. mineral oil.
You can get advice on anything related to this on AirCondition.com

The sight glass isn't an effective indicator with 134a, only works with R-12.
Yes, I think you should use R-12, the system was designed for it and it is more
effecient than 134a. To go back to R-12, you have to flush everything but the
compressor, replace the accumulator,run mineral oil thru the compressor to
flush the other oil and recharge to spec with R-12 and proper amt. mineral oil.
You can get advice on anything related to this on AirCondition.com