My understanding, for what it's worth: R134a doesn't work nearly as well as R12 in an unmodified R12 system.  Mods can run into some serious change. R134a will work, as your guy says; just not as well as what you have. 

Smarter listmembers than I may now comment ;-).

Karl Wolz
Sent from my electronic umbilicus

On Jun 11, 2013, at 1:58 PM, Jim Ogul <jimogul70@GMAIL.COM> wrote:

> My AC doesnt put out cold air on my 85 Westy.  It was that way when I
> bought it.  It has R12 in it.  A local shop tells me that what they do is
> evacuate the system and put in R134 and if the system is ok it will work.
> I had always heard you have to get into the system and modify it for R134.
> This local shop has a lot of experience and if its that simple I'd rather
> go that way.
> 
> Jim
> 85 Westy