I don't have much experience with breaks so please let me know where to start. Front disks and rear drums work fine with normal use, but after a panic stop, the rear drums don't disengage. I took off the rear wheels, and looked for leaking pistons, corrosion, or other problems but I don't see anything wrong. They appear to be adjusted fine, turning only one revolution while up in the air with the wheels back on and in neutral. Is there some other adjustment I'm missing? I heard somewhere that you have to drive in reverse and panic stop to adjust the rear springs - is that true? Could this be contamination in the lines or something else?
Any help is appreciated.
I gotta keep this thing on the road until I build by house and garage - otherwise its going to rot in the front yard before I can restore it.
Thanks,
-jack

Just a possible problem, the hose to the rear brake line may be swelling and causing your problem. Should be available at the local Napa store. Can you get it to happen while on jack stands?

The hose is one thing to check but one other thing is the backing plates. It happened to me on a Suburban and I just about rebuilt the rearend before I found it.
There are 3 flat rectangle areas where the shoes ride. They are supposed to be lightly lubed but usually aren't. On my truck, they had rubbed a groove right through. Everything would be fine until, I, like you, hit the brakes hard and then they would lock up. I could stand on the gas an spin one wheel like I had Line-Loc. Eventually, I could "rock" it loose by going forward to reverse and back.
What was happening is it would jump the groove and the springs couldn't pull them back. Look close because it was hard to see. I just laid a bead of weld on them and ground them flat with an angle grinder and they were good as new.

Jeff

Backing up and applying the brakes is what keeps the emergency brake adjusted on drum brakes. Panic stops in reverse, don't know about that. When ever new brake shoes or pads are replaced , you don't want to panic stop, till these brakes are broken in a little. Even after that you don't need to panic stop to adjust springs or for any other reason.

The backing plates being worn could also cause the brakes not to release, check to see if there is a groove worn there where the shoe sits on the plate. Good point.

Originally posted by Hi-po SS 454:
Backing up and applying the brakes is what keeps the emergency brake adjusted on drum brakes. Panic stops in reverse, don't know about that. When ever new brake shoes or pads are replaced , you don't want to panic stop, till these brakes are broken in a little. Even after that you don't need to panic stop to adjust springs or for any other reason. Actually backing up and hard braking adjusts the shoes, nothing to do with emergency brake. The twisting motion of the shoes when doing that pushes the adjuster down against the star wheel which spreads the bottom of the shoes apart. They usually take care of themselves if everything is cleaned and lubed. When things get dirty/corroded you may have to brake harder than normal, in reverse, to make them adjust. As far as the locking problem, don't know. You said they have about the right amount of drag. If they weren't adjusted out enough that could do it but sounds like that is not your problem. Both rears?

Once they are locked up try cracking the rear line at the master cylinder. You could have a piston sticking in the bore. If they are still locked up after the line is cracked then you've eliminated a fluid problem and likely something inside the drum as suggested. If they are loos then I think you have a Master cylinder problem. Kinda seems unlikely that both brakes would have a mechanical problem but I guess its possible.

Finally, may have hit on the problem, I have seen this problem before, check to make sure the brakes/adjusters are not on backwards (which is entirely possible because the backing plates are symetrical). this would cause the "panic stop" brake adjustment, that is normally supposed to occur only in reverse, to happpen while going forwards.

good luck

I should add that I actually did this to myself once on my drivers side rear and reversing the setup corrected it.