Scarebird uses those calipers with s10 rotors and brackets he makes to bolt on the drum spindle. I dont know on the rear rotors, but the frnt calipers are the same calipers the s10 uses up front, and the 80-85 eldorado rear calipers are basically the same as the 3rdgen rear calipers.

The kit is balanced but the brackets seem flimsy compared to OE, and the rotors and calipers are smaller too, but it seems to work.
I dont know why a master from an 81 either... an 82+ would be a better size IMO... since you are basically running a 3rdgen brake set up.
BTW those aluminum masters have no residual valves as far as I know.

69FBM is right on the money. You want to start loosening fittings and bleeders to check for restriction. Since it is all 4 wheels, loosen the fittings at the master, and if the brakes release, it is a problem at the master/booster. If not, start at the wheels and work your way up.
Loosen a bleeder screw, and if that brake releases, it is a problem above the calipers (hose, check valve, master, line...)
If the brakes dont release when you loosen the bleeder screw, the problem is at the wheel (caliper, hdwre, bracket...). It would be unusual for all 4 calipers or hoses to go bad at the same time unless there is a contaminant.


You might also try unbolting the master from the booster to see if the brakes release.