Mike,
My 44 11/2 ton has vacuum assist, while my '51 two-ton has Hydrovac. I make the distinction as follows:
The 44 has a large vacuum canister, connected to the manifold via vacuum line, which provides added heft to the master cylinder. BUT, it's vacuum assist only - no brake fluid is involved at all. In fact, I could remove the canister mechanically, and not much would change.
The '51 has a somewhat similar-appearing canister, but it is more complicated, as it involves the brake fluid entering and exiting the canister. The master cyliner pumps fluid to the Hydrovac, which then distributes it to the wheels (at least, I think that's the general idea). When you bleed the system, you have to bleed the Hydrovac. If I were to remove the hydrovac, I'd have to bypass it with the fluid lines to maintain any sort of braking ability.
The vacuum assist was an option on "heavy-duty" Chevs in the '40s. I believe the Hydrovac was standard above a certain level in the '50s. Regardless, it's a good thing to have - the general claim is that the Hydrovac does over 70% of the braking if properly maintained.
Now, all that being said, I have nothing to offer about a dual-master set up! sorry.
-Michael S.
