[jocko]

pedal dropping once truck is started (i.e. vac signal to the booster) could be a sign your power booster is bad/diaphragm torn internally. is it new as well? Still seems that is the main clue - that you have a solid pedal when engine not running and no not when it is - only diff between those two states is vac signal to the booster. Most of the problems alluded to above - and they are all valid problems and things that happen - would mostly seem to occur whether or not the truck is running. i.e. an open line or big air bub will compress regardless of engine vac signal, won't it? (and I'm not sure what the answer to that question is...)