ok first up from what I've seen park brake and brake pressure light are 2 different ones correct?
if so like I'm thinking, having air in the system has NOTHING to do with a PAK BRAKE LIGHT, but the actual BRAKE light was on then yes THAT is why, air in the lines. or pressure problem between the front and rear.
So now,,,, did you bleed from the PS rear, then ds rear, then do ps front and ds front? Do them all regardless it's not going to hurt, it will just flush the system.
Some say oh no the front and rear are seperate no need to bleed the fronts if only the rear had problems, but not me I do them all anytime.
Old dirty fluid can ruin things in the system and cause rust to work faster on destroying things.
Now you could have a number of things wrong, could be a bad whel cylinder, could be the rear shoes are not adjusted out tight enough or close enough to the drums, that happened on mine. The shoes could be worn too much as well. If it was me, change both wheel cylinders, about $10 a piece, inspect the rear hard lines closely, if any rust change both, the parts store sells straight lines, flared with the correct ends(at least for my 88 anyways they we corect) then the one problem mine had which I had no clue of was the rear flex line, it let fluid out and I could blow thru it, but after changing EVERYTHING, MC, prop. valve, wheel cylinders, hard lines etc. I still had crap brakes, mine was the hose and one side not adjusted out enough.
Also the ear dump valve on the earlier RWAL trucks is a problem, it can be bypassed, the complete ABS ones I'm not sure about, then of course you come down to a MC, and prop. valve which very rarely goes out, but could need reset or held open.
IMO if your rear brakes were fine and didn't blow, the prop. valve should be fine, I think the newer 4WAL doesn't even have prop. valves.
So check the shoe thickness,
adjust them out
then if nothing else,
change both wheel cylinders or rebuild the ones you have,
inspect and maybe change hard lines(doubtful it's any problem but it'd help reduce risk or bursting later)
then change rear flex line, about $17 or so.
If NONE of that fixes it, I'd get a MC and try it.