I think the problem is more the oil, and not the pressures.  Sure the
average pressure with R134a will be a bit higher (maybe 10%), but
because the R12 pressure switches are still in place the maximum
pressure will be the same.

Converting to R134a without replacing the oil is a problem.  The
mineral oil used with R12 is not miscible with R134a; this means that
the oil won't flow in the system to lubricate the compressor.  The
best way to retrofit is to flush the system and replace the mineral
oil with double-end-capped PAG or POE.  PAG is shown to lubricate
better than POE, but it's not as compatible with different
refrigerants; specifically double-end-capped PAG is the best at
resisting moisture absorption.

Without flushing the system it's possible to just add the replacement
oil on top of the mineral oil.  No ideal as it will reduce cooling
among other things but better than nothing.

Ryan

On Thu, May 9, 2013 at 7:53 AM, JRodgers <jrodgers113@gmail.com> wrote:
> Tom,
>
> Thanks for putting this out there. Probably the single biggest factor
> with a Vanagon in converting to 134a is the compressor. I find my van AC
> is adequate with the 134a. I have not had the problem- yet - but from
> what I understand, the compressors often fail when the 134a is used
> because they are not designed for the higher pressures that are required
> by the 134a.
>
> NEW later model Sanden compressors that fit the Vanagon and are designed
> for 134a can be had from the 'Bay" for around $200 or so. A while back I
> looked at one for $215.
>
> John