Marv,
I'm sure you know this already, but I believe the 41-46 big trucks only used the "Brake Booster", which is technically different than a Hydrovac. The Booster strictly uses vacuum to assist in the mechanical application of the existing Master cylinder. It's really just helping you push the pedal down.
The later trucks used an actual Hydrovac, as shown in the e-bay listing. These, to my way of thinking, are more complicated, as they include the slave cylinder, which actually does the work of pushing the brake fluid through the lines. It must be included in the bleed process. They are a bigger boost to braking effort than just the brake booster.
That being said, no reason I can think of you can't put an actual Hydrovac on the earlier trucks. I think a proper installation would include a check valve in the vacuum line, which is typically mounted on the fire wall, above the level of the carburator. The check valve, I believe, prevents "Chloro-Flourocarbons", or Dilithium Crystals, or Cholesterol, or some-such free radical, from filtering back to the Hydrovac, and eating away the rubber diaphram.
Either way, I'm told the most important thing, after bleeding the hyrovac, is to keep the air filter clean.
Good luck, and post about your results.
-Michael
