im slowly going through my new to me '91 to get it prepped for an upcoming DE. I went to check / change the rear pads and to my surprise the pad had almost zero material left on them. What I'm trying to figure out is do I need new calipers too?

the brakes were upgraded to "sport brakes" though I'm not sure from what year, or even if it matter? The front brakes look good, rotors look new and I just put on xp12. The rear rotors look ok on the front face but the rear face look like they've never seen friction. They're just a rusted mess. What I find odd is the back (inboard) pad looks more worn than the front pad (outboard). I would think if the back of the rotor looks like it hasn't seen a brake pad for decades then the inboard pad would be less worn.

So 2 basic questions:

1. Should I replace the rear calipers?
2. How do I know, or should I care what year the brakes are from so I can pick the correct rotor?

this is the rear face of the drivers rear rotor and matching pad.

 

If they aren't seized, they are fine. Lube them up and use good pads with new rotors.

 

Dear lord, is that your ******* rear rotor??? Looks like the Titanic wreckage.

Look, stock single piston brake calipers are based off of Newton's third law. As the piston pushes against the outer surface, the caliper should slide on it's pins, therefore pushing (pulling?) the rear pad against the back of the rotor. If your pins are bent, rusted, or corroded (I'm guessing they are), they'll be seized to the pins, and the only force on the pad will be from the piston side.

But to answer your question, judging from the photo you posted, you need new hard brake lines, soft brake lines, calipers, brackets, rotors, pads, and probably control arms, sub frames, and a new chassis.

 

 

I would guess that something is frozen with rust.

 

The piston moves with the allen wrench, if it were frozen would that still be possible?

 

No. The piston is not the problem. It's the pins. Get a kit from Rockauto.com

 

Based on the condition of your rotors, I'd replace the whole shebang...

 

Ordered new pins and rotors. It's easy enough to pop back off if it turns out the calipers are garbage.