If I remember correctly, R134 refrigerant is a smaller molecule, and hence more easily gets past seals than R12. 

On May 7, 2013, at 8:50 PM, Tom Hargrave wrote:

> Was it holding R12 fine? It could be the seal in the front of the compressor
> - that seal depends on a oil film to not leak. I've seen a lot of R12
> systems that barely leaked start leaking a lot when converted to R134 just
> because the molecule size is smaller & the gas passed through the compressor
> front seal easier.
> 
> Thanks, Tom Hargrave
> www.stir-plate.com
> www.towercooler.com
> www.kegkits.com
> www.grow-sun.com
> 
> 
> 
> -----Original Message-----
> From: Vanagon Mailing List [mailto:vanagon@GERRY.VANAGON.COM] On Behalf Of
> JRodgers
> Sent: Tuesday, May 07, 2013 3:50 PM
> To: vanagon@GERRY.VANAGON.COM
> Subject: AC 134a refrigerant leak down
> 
> 18 months ago I had the AC system gone completely through - and converted to
> R134a. It got new seals and the whole nine yards. the system ran great
> through the hottest months but then began to show signs of 134a leakage by
> virtue of reduced cooling. That got progressively worse. Twice I added
> refrigerant, but the cooling has once again dropped off.
> 
> Givben this - obviously I have a leak.  I was wondering about the
> application of one of those sealers to the refrigerant? Do they really
> work??
> 
> I dropped a bundle to get this fixed, and would hate to have to do it
> again.!!
> 
> Recommendations?
> 
> Thanks,
> 
> John Rodgers
> Birmingham, AL
> 
> 
> -----
> No virus found in this message.
> Checked by AVG - www.avg.com
> Version: 2013.0.3272 / Virus Database: 3162/6305 - Release Date: 05/07/13