Too much travel before brake booster kicks in?

[JLeather]

My '79 has too much brake pedal travel before it engages the booster. I thought it was the brakes out of adjustment but I checked and adjusted them (the rears anyway) and it didn't change anything. The pedal travels a good 3 inches with essentially no resistance at all before it engages the booster (can hear the booster sort of whoosh a little when it finally hits). After that it firms up pretty quick and the truck stops. Not sure if there's an adjustment I'm missing out on between the pedal and the vacuum booster, or if this is a sign of some sort of problem with the brakes I haven't properly diagnosed yet?