Personally, I did experience severe brake fade when using DOT3, especially
on mountain roads, but also sometimes around town (lots of hills in San
Diego). I rigged up a pressure bleeder and flushed out, replaced with DOT4,
brakes were fine.  This is assuming that Firestone actually replaced the
fluid as I paid them to do a few years ago, that I wasn't driving around
with 7 year old, water saturated brake fluid. They did manage to screw up
the rotors and put on the cheapest, smelliest brake pads I've ever seen,
but they wore out in 10K miles. That was, coincidentally, the last time I
ever paid someone else to work on our Vanagon.

I would spring for the extra bucks and put in DOT4.  The Vanagon is a
pretty heavy vehicle, the braking system is marginal to begin with, I
figure those few extra degrees of boiling point might be the margin I need
at some point.

Dan Fuller
'90 Carat