when I bought my 74 vette it had just been changed over to the r134 because of the cost involved. I have plenty of the r12 so if I pull the system to vacuum, add r12 oil can I go back to the r12 or is it more complicated than that. Also the air isnt strong from the vents so I need to figure out where the air is leaking because the fan has all the speeds. Any suggestions as I would like the AC to function properly. Thanks
Rodney

I know that the other way around is a no no.  I was told the washers (rubber/neoprene) and a couple of other things are changed.  I do not know if r134 system can take the r12 for sure.  If know one else has a fore sure answer I can try to get hold of a friend (we both lost our jobs at the same time about 5 years ago but I think I can find him) that did conversions and ask.  I have some R12 also but have not gotten around to seeing if my 82 air conditioner works yet.  I hope it does as the conversion is costly I'm told. 

Check with Terry (my7T1) PM him, he's in the  AC business.

The problem with doing this either way on the '74 is the VIR(valves in receiver) system. That is where a lot of the oil will be, and it's danged hard to get it out without removing the VIR, which is a major pita itself.
If your car has been converted to an orifice type system, then it ain't so bad. Either way, you'll need to drain the compressor of as much oil as you can, and replace the dryer(if it has been converted).
The seals used for 134 will handle R12, as far as I know, since they are better to start with.
Why do you want to go back with R12, anyway? I know it feels cooler than 134, but...if the system is working properly, it will cool just as well.
Getting all of the air to come thru the vents is a wonderful fantasy for most of the Shark cars. It can be done, tho, with a little attention to details under/behind the dash.
Most of the tubing connections were "close" from the factory, and "sealed" with small strips of foam rubber. Over time, the seals disappear, and now "close" ain't even in the picture!
I've seen vent connections that were at least 1" apart, with gaps you could stick your arm in(well...at least a couple of fingers...).
If you get all of the tubing connections sealed, it's a HUGE improvement in air-flow from the vents. You also need to make sure the air doors are all in the correct positions for whatever vent you want the air to come out of(heat/A-C/defog).
It's a major deal, but certainly worth the effort.

The efficiency is a bit different between R12 and R134A. R12 boils at -22 degrees F. R134A boils at -16 degress F. R12 uses mineral oil, R134A uses PAG oil. The "O" rings are different on the R134A due to the oil. The pressures are slightly higher on R134A. Now to your question; You can revert back to the R12. The oil will work as well as the different drier/accumulator. The only diffenence  would be in the compressor cycling switch. It will shut off the compressor clutch at a slightly higher pressure. It's hard to give more information to you without my knowledge of the total changes that were made from the original R12 to the R134A conversion. I'm not trying to confuse you, if you feel that you have the R12 and the time...go ahead and experiment with it. Hopefully you have a vacuum pump and gauges to do the work on the system.  I've never have known if anyone has done this.  corvette440hp2009-02-16 09:13:56

The R134a molecule is smaller then the R12 molecule.  Thats why you need better seals.   All 134a seals and equipment will hold R12 with no problems at all.

First question I have is, where did the refrigerant go? If the system leaks you need to find out where and fix it. From there I have to agree with Joel and Ken. Why would you want to change back? If the system was retro-fitted properly, I would continue with 134A. Doesn't seem like it would be worth the time and expense to change back. In my years of business, I have never heard of anyone retro-fitting a system from 134A back to R12.  However it is possible. The oil difference is the most important factor.