Brake problem?

You clamp the rubber brake hose, and the pedal is there, then you unclamp it and the pedal goes away?
There is air in the calipers if the above statement is true.
Dumb question, but are you SURE the bleeders on the calipers are at the TOP of the caliper?
These trucks are designed so there will be NO front brake action until the REAR shoes contact the drums. Having said that, make sure the rear brake shoes are adjusted correctly. There should be slight drag on the drums if adjusted right. Also, check the bleeder screw that is in the frame C channel for the rear brakes, above the axle, drivers side. There could be air in there if you didn't bleed it here.
If everything checks out, my educated guess would be that you got the wrong master cylinder for the truck.
Is the original MC still around somewhere? Maybe you can rebuild it, or at least get a number from it to verify the replacement is correct.
Also, there are 2 brake CALIPERS that were available in 90. Bendix & Chrysler. I dunno if the brake caliper style would determine the master cylinder used, but you might want to check into it.

Hope this helps some.

George