A Hydrovac unit is a vacuum booster that is mounted on the frame and the assist part is activated by the brake line hydraulic pressure. These units are very common on the single pot MC systems. What your talking about is a modern power brake booster that is mechanically linked to the brake pedal and has the MC bolted to it (these units are not call Hydrovac units) and they are very common on dual pot MC systems.
No matter what type system you install you need to use the same bore size as the original MC had, otherwise you won't move enough fluid out into the system.
Hope this helps,
Mike B
