Symptoms:
Sometime the pedal seems just fine, others its rock hard. Doesnt seem to be any particular situation it does it, purely random.
Troubleshooting:
Unplugging the vac. line to the booster and there is quick, strong vacuum. Plug the booster in and test somewhere else the vacuum is no where as strong and takes considerably longer to build. If you have someone working the brakes the vac. will not build to even half of normal.
Diagnosis:
I think its the brake booster. Guy i work with had the same problems with his 91 and confirmed that it was the booster.

However the booster does not make any iregular noises, which in my experience is common with bad boosters.

Now, i need some help here. I can replace the faulty booster with NAPA AUTO PARTS
for $79, or i can replace / upgrade with https://www.ford-trucks.com/forums/1...st-brakes.html

Roughly a $325 difference. Truck has new calipers and pads, new MC and front lines. Rears are adjusted. With a working booster the truck stops great and has good pedal. So its hard to justify spending the extra coin.
I love the brake pedal with vacuum, as far as you barely have to put any pressure on the pedal to stop, (our 94 you could pretty much lock up all 4 on pavement with a finger). I do not care for the hard pedals like in our 03 superduty.

 

If vac is good when the booster isn't hooked up, it seems pretty conclusive to me.
I have no experience with the cheap boosters, although NAPA generally is pretty good...

 

Lifetime warranty not sure if that is indicitive of anything or not. Usually like you say, theyre pretty good. Just stay away from thier electric stuff (starters, alternators, fuel pumps) and i was told recently not to use thier u-joints as the crosses are not case hardened.