[dsraven]

have you checked the booster for a vacuum leak or possibly a bad vacuum check valve at the booster?
pull the check valve out of the booster, disconnect the vacuum line and do a test to ensure it allows vacuum to flow one way and absolutely nothing goes back the other way
assemble the valve back to the booster like it was. now do a vacuum test on the booster to see if it holds a vacuum. simply apply a vacuum to the check valve and monitor to see if it drops in vacuum and listen for any air leaks at the inside of the truck around the booster pushrod, also around the grommet for the check valve.
question, engine idling, apply the brakes. does the engine rpm change at all? like it had a sudden vacuum leak to make up for?
is the pushrod from the pedal set up with some freeplay so as not to apply a slight pressure to the booster pushrod which would unseat the internal valve and "leak" vacuum?
maybe google how a vacuum brake booster operates for a little more theory on what could go wrong