The Stovebolt Site Forums The Parking Lot Big Bolts Ideas on brake failure theory

The brake vac line should be pretty beefy really, and there should be a check valve between the brake booster and the engine to prevent backflow of pressure into the booster. It MIGHT concievably give you one good pump on the brakes before it eaqualized & you lose pressure.

What I experienced with my old beast when the brakes failed (the booster was inop as well, but they did work when pumped up) was that the brakes were adjusted poorly so that when I let the pedal come all the way back up, the master cylender would release the back pressure off all of the slave cylenders and the shoes would retract far enough that they didn't contact the drums without a few quick pumps.
I've since rebuilt almost all of the system and properly adjusted the clearance on the shoes. Even as it sits there's a big difference, driving it will show a marked improvement I'm sure.

I wasn't as fortunate as you though, mine decided to let me know who was smarter (not me evidently) as I was aproaching a red light at a busy intersection,... :o

Oh yeah, pucker factor 12 that day!

Good thing about being in low range with a drive shaft brake, it was more than enough to stop the truck. Needless to say, I decied it was better to pop for the tow home.