Most trucks use air operated brakes, climb back in your hole wombat.
I was making a point that trucks dont use blow off valves because they dont have a throttle plate, what happens when you open the throttle completely on an aspirated engine?? Manifold vacuum disappears.
Thanks for the education though, ill keep it all in mind. As Wogboy said, once you create a restriction the vacuum is huge.
Getting back on topic now....