[dsraven]

-LG makes some good points. you never know what you have until you take it apart and check. sometimes that is faster than racking your brain troubleshooting or checking the net for how someone else fixed their problem which may not be the same as yours.
-a lot of the brake backing plates have an inspection hole in them for each brake shoe so the lining thickness can be checked. some may not be actually punched or drilled out but are simply a divot stamped into the sheet metal backing plate. these can be drilled out to see in there and then a rubber plug, like the one that covers the adjustment hole, can be used to fill the hole after. this would allow you to see what is happening at each wheel when a helper steps on the brakes. are the brake shoes moving and retracting properly? of course a drum removal and system check would be the best but this gives a quick indicator with little mess (well, unless you gotta drill the holes out).
-you might also check the backing plates where the brake shoes rub as a lot of the old used plates have worn spots where the brake shoes rub agaist the backer. worn enough so the shoes drops into a small groove worn in. this means the shoes may not move out of the groove properly or may not retract properly so they are difficult or impossible to adjust correctly.
-curious, with the booster vac line unplugged and capped do the brakes stop the truck? have you done a vac check to see what you have for vacuum on the booster line at the booster?
-what master cylinder is being used? bore size, previous use (drum/drum, disc/drum, disc/disc). was the master cyl and booster a bolted together unit from a donor or a mismatched set that was not actually used yet as a set when bolted together? this goes back to LG's comment on correct set up of the pedal to booster pushrod adjustment, booster to master cyl pushrod adjustment etc. is there a combination valve or proportioning valve in the system? which brake line circuit is connected to which port on the master cyl? if you remove the reservoir cap and have a helper slowly step on the pedal while you look (safety glasses) at the fluid res, is there 2 little fountains or fluid that arise when the pedal is first stepped on? this little check quickly lets you see if the master cyl pistons are returning fully and allowing fluid to return to the res and also allow fluid to replenish the bore. the pistons retract back behind the bore ports so when the pedal is first pushed the pistons move forward and fluid is moved but it takes the path of least resistance, which is simply back into the res, until the pistons pass the port and start to push fluid down the brake lines. pressure in the system starts to build when the linings contact the drums and the pedal pressure is maintained.
-I would say do a few checks and then if the problem isn't obvious, like mismatched parts or out of adjustment then maybe pull the drums and see what you have to work with.