Could I have lost a seal going into the Vacuum Assist??>> Yes

Any way to easily inspect the most likely areas??>> You will have to remove
your brake MC to see if you are leaking into your vacuum brake booster.  I
would go around the van again and check over everything.  You could also be
loosing fluid from the rear wheel cylinders, and you will have to remove
the drums to check these.  The rear wheel cylinders leaking is more common
than leaking into the vacuum booster, but both happen from time to time.