Tom>
It looks more complicated than it is. the Hydrovac uses the vac port on the manifold to suck the air out of one side of the can. When you press the brakes there's a little piston in there that operates an air valve that lets outside air enter the vent tube on the opposite side of the can allowing the engine vac to draw the piston down. Piston is hooked to another piston in the small end that pushes your brake fluid.
Once you release the pedal the air valve closes and the pistons return to thier original position. The back brake pressure is released by a simple valve that opens when the piston bottoms out so you dont have dragging brakes.
It's very similar to some of the valves I work on in aircraft, hence why it's built by Bendix Aerospace.

1956 GMC 370 dump " 'Tater "
1970 VW Volksrod "the Black Bomber"
2007 Chevy Avalanche
2020 Jeep Cherokee Trailhawk,
2005 Harley Davidson FLHTCUI

I dig all cars, old and new, whether they were hammered out of American iron, German steel, or Japanese tin cans. Being unable to appreciate them all is missing out on a world of great things.
But thats just MY opinion! :P