sounds crazy but pull the boot back on the top of the mc and fill the back side w brake fluid. works about 2 out of 10 times. all it does is improve the backstroke suction and sometimes thats all it takes.
mrfixit is exactly right, you have to bleed theses from the bottom up, and it also helps to bounce the pedel while doing it too. I recently replace a defective M/C on one of these, it was only a few months old, I realized that the M/C was defective when I tried to bleed it as described, and I couldn't get any fluid to flow up past the master cyl. Also I would advise to never push the pedel all the way to the floor until the system is fully bled.
Those things are real tricky to bleed. The best way I have found is to get a new pump type oil squirt gun and fill it with brake fluid. Hook it up with a hose to the bleeder on the wheel cylinder and pump it. Pushing the fluid in and UP to the master cylinder gets the air out. BTW that has to be one the worst brake/inching systems going.
Not from my experience...talking while I was around someone fixing this same series truck...The adjustment in the pedal linkage is very critical...