Stan, 
I have to agree with the earlier post 134 is not as good in an R12 system .

Check out the article on Go westy's site here  

http://www.gowesty.com/flow-thru_ventilation.html

Snip from the article below.

The problem is that R134a has about 80% the cooling capability of R12. A/C systems designed to use R134a have a different, much more efficient manifold type "parallel-flow" condenser as opposed to R12 systems "series-flow" condenser. These "conversion kits" are noting more that a set of fitting adapters, and R134a refrigerant and oil. On the late model non-pop top Vanagons simply converting to R134a works fine because these vehicles have more than enough capacity. So giving up 20% efficiency is not big deal. But on pop-top Vanagons were the A/C already sucks, it just is not an option. Eventually when R12 is completely phased out it will be necessary to properly convert these vehicles, which would include replacing the condenser with the more efficient type. 
  ----- Original Message ----- 
  From: Stan Wilder 
  To: vanagon@GERRY.VANAGON.COM 
  Sent: Wednesday, June 16, 2004 12:26 PM
  Subject: Re: R12 to R134a Air Conditioning Conversion

  On Jun 16, 2004, at 11:35 AM, Mike Miller wrote:
  >
  > Since the AC is marginal for our big boxes with R 12, it's even less
  effective with R 134a.  I'm thinking of adding a second condenser under the
  van.
  ---------------------------------- Clip -----------------------------------
  Don't think about it, just do it.
  I put one under my Porsche and the A/C is so cold you can't keep your
  fingers in front of the vents when you're on the highway. Temps around 34
  degrees are normal from all vents with just a Mini York compressor and R134a
  refrigerant.

  Stan Wilder