I am no AC guru, but with all this talk  about 409, I figured I should pipe
up.  First, 409 is not approved for mobile systems.  It was designed for
home, stationary systems (which is why it is sold only in 30lb cans).
Therefore, it cannot legally be used in your vanagon.  Also, it is a blend
contianing something like 60% R22 .  As a result you will see extremely high
head pressures using this stuff, especially at high ambient temp.  I realize
people have used it, and I am really not saying not to or that it won't work,
I am just trying to point out that it is not the holy grail of R12
replacement  and if you want to play with your AC, you should do the research
and be comfortable with what you are doing.  An AC system is one of the few
system on a vanagon, that can acutally hurt you.    My. .02  Charlie..