FWIW, I see this thread is old, and I'm no brake expert, but here's what I learned:
When I first got my M1008 it had approx 65,000 miles on it and weak-ish braking and a not very solid feeling. Not at all like my two civvy WT square bodies (1977 C20 and 1986 GMC 3500) with the same type brakes. Both those trucks would stop on a dime! I had completely rebuilt both of their brakes with all new brake hardware, over a period of time. Yeah, that cost a me a bunch, but considering the liability of bad brakes and an accident with my CUCV, it's damned well worth it!
Fix for my M1008; After doing parts search, I discovered there are two different part numbers for the CUCV brake boosters; one for M1009 and another for M1008. Pulled my 1008's booster and discovered it had PN for an M1009. A lot of eBay sellers claim the boosters are interchangeable. Anyone, please feel free to correct me if I'm wrong, but this may not be true.
RESULT: so I searched and found a surplus GM NOS booster w/PN for the M1008 and replaced it with my drums and rotors too. Also Installed steel braided brake lines up front and new stock lines in the back. Bled the system w/MightyVac, filled w/new DOT 5, now, an emergency stop in Old War Horse will put you through the windshield!
Another point I think should be made is IF the condition of your vehicle's drums and rotors are too far beyond tolerance to be turned, throw 'em away! Get new ones! THAT may be giving you the low-pedal feeling described earlier, especially w/worn-out rear drums. Mine were at their life's end so I bought new.
Sorry I don't have the M1008/M1009 booster part numbers anymore; this was like 10 years ago. Hope this may help somebody tho...