re:  he had installed Ronal 16" rims and done a rear disc brake
conversion with Audi 4000 Turbo front brakes

Very few automotive applications have bigger brakes in rear than in
front--I don't know whether Audi front brakes are bigger than Vanagon
front brakes, but do offer that Vanagons have a front/rear
proportioning valve, and if that's unchanged there may be no
difference anyway.

At the MSDS website, you can purchase "bullet train" brakes, which is
an upgrade of the fronts.  I can't get the website to work today, but
it's ...
http://www.msdsinc.com/vanagon1.htm