'97 3500: funky brakes, bad ABS maybe?

[AyWoSch Motors]

Got my 97 3500 running the other day after about 7-8 years. I posted another thread about its previous issues. Runs great, I'm super happy.
... anyway, during its test drive, I found out the brakes are all sorts of jacked up.
They lock up super easily, and do this weird chattering thing.
It you step on them hard, it just locks up the rear wheels, and chatters to a halt, and makes a funky noise and smell.
If you press super gently, itll pulse and lock up the rear wheels in little stages.
If you slowly press the emergency brake to stop, it works just fine, but of coarse only the rear wheels.
With its parked, if you press brakes normally they feel okayish, but if you press on them hard, the pedal goes all the way to the firewall and makes a hissing moaning sound. I pumped them about 20 times, didnt change how it felt at all.
It has plenty of fluid, so I know it's not that. It doesnt leak at all, even when being pressed.
My first guess is the ABS has crapped the bed. Or maybe it just need the be bleed from having set for so long. I dont know.

What do you guys think?

 

[bucket]

There could be a combination of issues. The first things that come to my mind are heavy rust patches on the rotors and drums. Could have leaking rear wheel cylinders too.

 

[AyWoSch Motors]

There could be a combination of issues. The first things that come to my mind are heavy rust patches on the rotors and drums. Could have leaking rear wheel cylinders too.

That's possible about the rust. It did sit a long time.
There is no evidence that the wheel cylinders are leaking, everything is nice and dry.

I took it for another ride today, out on my main road. The more I use them, the better they seem to feel. I was doing about 70 and stomped on them pretty hard, and it didnt lock up or make any noises. Slower down pretty well.
And when your at slow speeds now, they work pretty well, but if you press a little too hard, they still lock up.
Might just take time. And it might also be that there is almost no weight over the rear axle, so the rear tires are happy to just lock up and get dragged.
I also noticed today, that its only one wheel doing it mostly, the drivers side rear.