I would suggest you pull each wheel and drum and do a visual inspection of the W/C pistons. I say this because if one side of the cylinder is spreading and the opposite side is not, you will get enough pressure from the one side to stop the wheel from turning.
You will need a sledge hammer and a propane torch to remove the drums unless they have been removed recently. Remove the wheel then remove the drum retaining screws from the front side of the drum, your torch may come into play here because those retaining screws have a tendancy to rust tight to the drum. I use an impact socket driver for rusted screws,one of those you hit with a hammer,that seems to work well for me. A word of caution, dont overheat anything and try to keep the heat directly toward the screw or part you are trying to remove.Loosen all adjusters so you can spin the wheel freely with zero drag.Heat the front of the drum where it meets the axle(between the lug studs and around them)and tap on the back lip of the drum with the sledge all the while keeping the heat on the front of the drum.When the drum breaks free it will most likely come right off if you have the adjustment backed off far enough.
That double diaphram booster has no bleeder(at least I don't beleave it does)but there should be a fluid fill port with a plug and you will need to put a pint of fluid in that fill hole as part of your bleeding procedure.
Best way I have found to do that is by opening the bleeder on the passenger rear wheel and have an assistant pump as you add fluid to the booster.You know the drill,add fluid,open bleeder,pump,close bleeder,pump and release pedal,add fluid and so forth until you get that booster to accept a pint or so fluid.
My big truck has no bleeder at the booster but has two on each of the rear wheels,one for each of the two wheel cylinders per side.
The shop manual just says add one pint of fluid to the booster but gives no instruction for getting the air out so it will accept the fluid.
I would check the connections on that booster because you said it had been replaced. There is a possibilty that the lines were connected in the wrong place.The quick check for this is to have someone pump the brakes and hold pressure while you crack open what should be the line going to the rear of the truck and see if you lose pressure at the brake pedal,if you lose pressure then all is okay with the connections.
Be sure and check the adjustment on the brake linings by getting the wheel off the ground enough so that you can turn it by hand. Adjust the brakes tight until you cant turn them by hand at all,then back off that adjustment until you can turn but still have some drag on the wheel..Remember, that truck weighs a bunch and what you normally call drag means nothing when that truck is rolling, so you want more drag than normal. If you have a brake adjusting tool the stroke toward the axle loosens the adjusment and away from the axle tightens it.
To futher help diagnose the problem I would need to know if you have AOHB(air over hydrolic brakes) Full Air or Vacuum Assist.
Good luck and I hope I have helped a little.Hobert
