BTW,
On my truck, at least, technically, it's a "vacuum assist", NOT a Hydrovac, at least not in the most common usage. Most people, when they say hydrovac, mean the unit as described in the tech tip listed above, which contains brake fluid, and is basically a second, stronger, Master Cylinder.
On my Art Deco 1.5 ton, it's a very simple vacuum cylinder that just helps physically push the master brake cylinder, no other brake fluid involved. I've always liked it, 'cause it doesn't complicate the bleeding procedure!!!
Either way, they use manifold vacuum, and the line is supposed to rise above the level of the manifold, I think so gasoline fumes/condensate don't drain into the vacuum assist/hydrovac and ruin the seals. (I think).
(If I'm right, I'm just showing off. If I'm wrong, I'm just asking for clarification!
)
-Michael
