Hi John,
When I converted to R134a, I replaced all the o-rings
with the new green ones. I took the old black o-rings
to an a/c shop and had them matched with green ones.
They usually only cost a few bucks. Better safe, than sorry.
I did my conversion back in May, and have no leakage so far,
and the system still works flawlessly.
My original post about my conversion is in the archives under
Re: a/c rebuild and retrofitted to R134a on 5/8/98 author "harald.nancy".

Harald
'90 westy

John wrote:
 > When I bought a
> reciever/drier from FLAPS I got green O-rings as I expected, then I went
to
> buy the rest from VW (ludicrous mark ups) and have been supplied black
> O-rings.  Anyone who has been
> supplied a green O-ring for a van or anyone who has used a black one
happily
> for a year or so without major leakage, please help???