Tim got it right, but I'll add this...
When stepping on the brake pedal you put pressure on the entire brake system via the Master Cylinder. This pressure also activates the Hydrovac unit which uses engine vacuum to amplify the pressure in the brake line to assist in stopping. If memory serves me the Hydrovac unit can put an additional 900+ pounds of pressure in the brake lines.
The rest of the system is just normal brake stuff (steel and rubber lines, wheel cylinders and brake shoes).
The Hydrovac will be a large canister hanging on the frame. Inside the canister is a piston that is attached to a small hydraulic cylinder. When the brake peddal is pushed, line pressure triggers a control valve on top of the Hydrovac to open engine vacuum to the large canister allowing the outside air to push on the back of the piston to assist in adding pressure to the brake lines. So, your Hydrovac will have a large fresh air hose (could be connected inside the cab to a filter), a vacuum hose from the engine (may be tied to a vacuum tank as well) and the hydraulic brake line off the hydraulic cylinder.
Hope this helps,
Mike B
