The Stovebolt Site Forums The Parking Lot Big Bolts is this the way my brake pedal should feel with hydrovac??

Dual resivior and dual boosters?... Wow, whats the GVW on that ol thing? Thats a lot of oomph!

I work with similar units on aircraft, mostly made by Bendix oddly enough, just for different systems is all. Seems to give me a little more insight into the operational design of them anyway,...

If I had to guess, & I do, I'd ask if you made sure you bled the air out of the tops of the HydroVacs where the pneumatic relay is?
Looking at how the system works, if there were a good sized bubble up there it would reduce the reaction time of the relay & thus when & how rapidly the booster engaged, hence the soft pedal into hard braking. Rust in the relay piston bore making it hang up could do the same thing.
Might also be a simple case of needing to adjust the slack out of all of the brakes & trying again. The relay piston relies on back pressure to move it, if there's too much travel in the brakes the fluid takes the path of least resistance until it builds up, so it would take more push before it moves.

Again, I'm not terribly expreinced with these old hummers, just offering my ed-u-mikat'd observations from other similar units & rebuilding my HydroVac system from the ground up.