Brakes locking up

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puregts

puregts

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I've run across this on two different cars both doing exactly the same thing, get this:

At low speed, just off idle, the brakes are locked up. As near as I can tell it is the rears. Putting throttle to it rocks the car but won't let go. Switch to reverse and car will roll. Switch back and you get about a foot or two and it locks up again. I'm pretty sure it's the rear as it won't power brake where normally it can easily.
Had the same thing happen with a Duster that had a disc brake conversion also. Both were treated to new wheel cylinders and multiple master cylinders (73 Duster disc/drum cast iron master cyl's).

Is it possible the distribution block that houses the warning light has failed? As in the shuttle valve that senses between front and rear pressure lines gets stuck favoring the front and blocking the rear? Is that even possible, because in both cars it does release but not before your frustrated. Having it happen while driving is a whole other experience at a few stop lights and folks behind you not knowing what you're dealing with only add to the fun.
 
I had that issue long ago, because the pushrod to the master cyl was too long. My fault, turned it in at the threaded adjuster back about 1 1/2 turns. There's a way to measure, in the FSM. Bad brake hose can do it also. The distribution block is brass, not much to corrode inside; although old fluid could carry some rust deposits there I suppose.
I think there's a more likely answer.
 
Be sure to check if the backing plate of the drum shoes has grooves warn in them. It should be smooth so the shoes move freely. That can cause them to catch funny (and stick).
 
What proportioning valve is on the car? Could be that I have a Duster with disc brakes here doing it but the brake light it on. I bought a rebuild kit.

Also as stated above the push rod being to long would cause that problem. Make sure you use the correct rubber lock to hold the push rod in place.

One more thing if you have a after market master without the threaded holes to hold the dust boot bracket there is a plastic spacer that goes on before the master to hold the boot. It does shim it out a little. Loosen the nuts a couple turns that hold the master cylinder and see if the problem goes away Here are pictures of the two different correct valves. Rebuild instructions and a picture of the master shim/ boot holder. I just received another on the dart we stripped

1970 - 1976 Mopar Combination Valve Rebuild | Muscle Car Research LLC

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DSCN1700.JPG
 
Just throwing this out there.

Have you verified that the rear shoes are on correctly

Have you verified that the adjusters are on the correct sides. The adjuster should work by applying the brakes when in reverse, that causes the lever to lift and fall back onto the adjuster wheel, causing it to turn a little if it needs to. If they were on the wrong side of the car they would adjust tighter every time you applied the brakes moving forward and become too tight.

How do you get the rear brakes to release? For more than a few feet?
 
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