Most likely junk in the "G" valve or the master is not returning to rest all the way. Also, old brake (not break) hoses should be replaced. All the brake components are sensitive to even the smallest junk. Hoses break down internally and as a result the fluid becomes polluted with small rubber and often has a grey color.
Jack the rear under the differential , not the crossmember, and bleed the rear calipers. If it frees the wheels you know there is blockage.
Also the factory service manuals should be read. They explain how everything on the car is serviced.
1980 SWII LRL-41587C FInj
1976 SS LWB LRE-23114 Harkness RR