When I opened up the Hydrovac on my truck I was surprised to find the guts were very similar to the innards of the pneumatic valves I worked on in aircraft that used small impulse pressure to operate huge valves. But then again, it's not uncommon for aerospace innovations to be used in the automotive industry, so,...
I think that hydrovac also allows for larger volume requirements of large trucks with multiple slaves per wheel &/or dual rear axles without having to mount increasingly larger vac assist master cylinders on the firewall or under the floor.
Just my thought anyway, I'm frequently full of it, ask my wife.
