Blue 48,
Suggestion: go ahead and replace all your brake tubing now; flex-hoses too. It's worth the peace of mind! This is a lotta truck; "no brakes is no good!"
I've had enough "loss of brake" experience that I won't even think about taking my '55.1 1-1/2ton out on the road until those brakes are completely re-plumbed!
Mikestem,
Sounds like your truck might have an earlier type of booster which is a straight vacuum booster on the pedal linkage; I've seen brake linkages on the pre-'47 trucks with an "extra eye" on the clevis that looks like where a brake booster can could be connected.
The Hydro-Vac was a Vacuum booster and Hydraulic cylinder that was connected "in series" between the regular master cylinder and the wheels; it "senses" hydraulic pressure in the system and then triggers the booster, and there's a series of check valves which make it all work correctly.
The nice thing about a Hydrovac is that they were a "remote unit";the only connections needed were Vacuum from the engine and place to "splice it into" the brake lines.
Chrysler & **** used similar systems on their cars in the '50s, mf'd by Kelsey-Hayes, and Midland-Ross, repsectively.
The vacuum section on the Hydro-Vac on my 55.1 still works; when I was running the truck around the yard last summer, I'd hear the thing grunt & groan as I operated the brake pedal(The air cleaner for the unit is inside the cab on the right side cab corner!).
I will probably by-pass mine while I'm re-doing the brakes until I can afford to have it professionally re-done; I don't plan on any heavy hauling for a while anyway.
I've looked at the shop manual illustrations and I'm in no hurry to pull this puppy apart!
