I will +1 what Kenneth said. Fully sorted stock brakes do an admirable job of stopping my Westy Syncro. Keep in mind that although rear brakes do not contribute much to overall braking performance, they must be 100% correct for the full system to work. This means correct brake shoes, free adjusters, correct brake cylinders.  Incorrectly radiused shoes or cheap rear cylinders and the master cylinder must provide more volume before the system pressurizes (Think of taking up extra slack). Some drum brake system use a residual pressure  valve to counter this.  I encourage you to research rear brake cylinders for Vanagons.