I have an '88 that has no rear brakes. The brake pedal is very firm, almost like there is not power brakes. I raised the rear spun the tires and nothing, they just coasted to a stop. I bled the rear brakes and do have fluid getting to the wheel cylinder. I can use the parking brake and lock the rear wheels up. I then adjusted the shoes to where they rub slightly and still no brake action using the pedal. I removed the drum and spun the wheel to see if the shoes were moving and they were not. I will add that the brake reservoir for the rear brakes was empty and it does appear to be holding now. I got the Jeep from my parents that did not use it very often. Wheel cylinders? Proportioning valve?
Thanks,
Brad

Could be plugged lines.

Your wheel cylinder pistons are stuck. New wheel cylinders will set you back about 25 bucks for both of them.

Reread your post, your wheel cylinders are frozen. New wheel cylinders will fix your problem, wouldn't be a bad idea to replace all the hardware in the drum.

I got curious so looked and found this:

"prop valves have a built in safety feature that blocks flow in the event that you blow a line. like if you blow a rear line, the prop valve stops flow to the rear lines so that the front brakes still work. if I remember correctly, there is a little rubber cap on the prop valve, pull that cap, and there is a little thing that you push in to reset it"

Somehow in lots of years I've never needed to deal with one, let us know what you find!

(one other said you just stomp it hard, I wouldn't know)

Just looked. Book says a force of 450 psi resets the "combination" valve, (after the fault is repaired)

I've blown brake lines and never had the proportioning valves do that. I would make sure you can bleed the brakes, then replace the cylinders if the fluid flow is good.

Well it ended up being the wheel cylinders. I replaced them today. What a difference brakes make!

All four pistons seized at once. Even though everything pointed that way it sounded unlikely. Wonder if it got backed into salt water launching a boat, then parked? Anyway, well done!