hey, my dad has a 1998 crew cab dually 4x4 454 now with 48,000 miles on it. i've asked around and can't find any info on this brake problem. we've bled them a million times and taken it to the dealership in which they found nothing wrong. we even put a new master cylinder on it tonight with new performance brake pads- nothing new.
there's about 2" of dead pedal before the brakes start to kick in. after that, it's still extremely spungy. plus, the pedal can go all the way to floor...my dodge can't do that. my friend's chevy's have mushy brakes too, but none this bad. sure, it'll stop, but it's pretty scarey towing a trailer. the ABS also seems to shake the truck rather than slowing it down (like my rear abs does). we also noticed tonight that the ABS doesn't seem to kick in (it work last winter)- fuses are ok, and no check engine/abs light(s) are on.
the only other possible options that i can think of have to do with the booster or the abs system being bad. any ideas 
Tom
1998 reg cab 2500 4x4 CTD 12v with some pretty descent mods..just ask.
