okay guys...so I've rep;aced my master cylinder and bled the brakes starting with the one farthest from the m/c all the way up to the one closest. It'll pump up nice and firm until I start the truck and the vacuum builds up...then it's to the floor with the pedal. What is up?!?!?!? Bad wheel cylinders and calipers??

 

I had the same trouble, and it turned out to be the vacuum booster.

it would work ok the first couple tries, but very touchy, then it started going to the floor. I found out the booster was bad after I took it for a 15 minute ride, and I heard air leaking when I pushed on the brake pedal.

It was hard as a rock when the truck was off. Killer thing is, it was already replaced with a reman. I grabbed the OEM from my '84 and swapped it on (took about 15minutes) and it worked great from there on.