You are almost definitely losing it through the Hydrovac and into the intake manifold if you can not find any leak source elsewhere.
If you lose it when the engine is not running, then it's still weeping out somewhere, but if it is losing fluid when the engine running and you can't find wet wheels or puddles, the Hydrovac has failed and is back siphoning the fluid into the intake and then it's burned off. Running a little rough at all? Sounds like you got a pretty good leak to pull down the entire system in a short period of time. How long did the truck run before you lost everything?
If you use DOT 5 fluid, well...nevermind.
Pull that Hydrovac unit and have it rebuilt and I think your problems will disappear. No I don't know where to have that done but there are tons of brake rebuilders out there and I"m sure many can do this for you.
Let us know the final outcome.
