Thanks everyone for all of the suggestions! Just as an update, before changing the master cylinder I lifted the dump to soak the 4 bleeders in the rear end in PB blaster. Upon inspection, the vacuum plate on one side has effectively been destroyed by rust. Something not all that surprising for a truck that has spent all 42 years of its life in New England. That being said, I have put the master cylinder project on hold. I now need to locate part numbers to replace all of the rear brake components. I am still going to attempt to bleed the system anyway to check the integrity of the rest of the system.
So if I cannot find part numbers for the parts I will have to disassemble the rear brakes and bring them to Fleetpride to see if they can match them up, that is unless someone has a better suggestion.
I will create a photo sharing account tonight and get some pictures it up!
