Check that vacuum line to the booster for condition / cracks, dry rot, etc, and make sure it seals good.
Did you lose brakes all together or did the pedal just get really hard and the truck didn't want to stop? If the latter, it most likely has to do with the booster...either it's not getting vaccum or the booster itself could be failing.
As for dragging calipers...check the pads for goofy wear...check wheel for any "chuck" to see if you have wobble in the wheel bearings, and make sure your caliper bolts are tight.
Dumb question...hows the brake fluid? did the brake light on the dash come on?