Rear brakes still locking up

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ro23dart

ro23dart

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Ok, ill try to make this short. I have an 8 3/4 SBP rear end in my 72 Demon. It was an 318 3-speed but i don't know if the rear was swapped in or original. The rear drums are 10 inch and i replaced the wheel cylinders because the old ones were shot. Now i can't get them to stop locking up when i brake firmly. The replacement cylinders are 15/16 bore. I tried a prop valve but i am afraid the adjuster is going to fall out since ive turned it so much. I also am sure that i have some air in the lines, i get a nice fountain out of the MC when i pump and release the brake. Question, can improperly bled lines cause the brakes to lock up? Also, it seems like the warmer the brakes the more it does it. I am planning on replacing the lines from the dist block all the way to the rear wheel cylinders, what else should i be looking for?
 
ro23dart said:
Ok, ill try to make this short. I have an 8 3/4 SBP rear end in my 72 Demon. It was an 318 3-speed but i don't know if the rear was swapped in or original. The rear drums are 10 inch and i replaced the wheel cylinders because the old ones were shot. Now i can't get them to stop locking up when i brake firmly. The replacement cylinders are 15/16 bore. I tried a prop valve but i am afraid the adjuster is going to fall out since ive turned it so much. I also am sure that i have some air in the lines, i get a nice fountain out of the MC when i pump and release the brake. Question, can improperly bled lines cause the brakes to lock up? Also, it seems like the warmer the brakes the more it does it. I am planning on replacing the lines from the dist block all the way to the rear wheel cylinders, what else should i be looking for?
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Have you tried adjusting the shoes themselves?
 
What is on the front? Are the bleeders "up" at the top of the cylinders/ calipers? You can NOT expect good performance until you get rid of air
 
Figure out if it is a mechanical or a hydraulic problem. When it locks up loosen the bleeder screw. If it's hydraulic that will release it. That would be step 1.....

JW
 
Front and rear drums have max dia 10.090 stamped on them. As far as locking them up and checking the hydraulics im not sure on how i would go about that. I guess i could have a buddy waiting with a wrench...
 
I've tried adjusting the shoes several times, it hasn't seemed to make to much of a difference.
 
ro23dart said:
Front and rear drums have max dia 10.090 stamped on them. As far as locking them up and checking the hydraulics im not sure on how i would go about that. I guess i could have a buddy waiting with a wrench...
Click to expand...

Put the car on jackstands. Load the rear with the motor and keep hitting the brakes. Based on what you are saying it won't take much to lock them up. Cut the car off, verify the wheel won't move with your hand and crack the bleeder. Simple, safe, and gives results.

JW
 
Replaced wheel cylinders but what about shoes and drums? Glazed over or shoes that have been soaked with brake fluid will act like what you're describing.
 
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GeorgeH said:
Replaced wheel cylinders but what about shoes and drums? Glazed over or shoes that have been soaked with brake fluid will act like what you're describing.
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Didn't replace those, they had plenty of pad. I will do that though, i would rather shotgun it and make it safer then fiddle with what i have.
 
ro23dart said:
No, hadn't thought about that. It seems to engage and disengage no problem.
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The problem with the cables is they could be too short or adjusted too much causing the equalizer bar to spread the shoes just a little. I had this on my Cuda and drove me nuts, as brakes would adjust nicely and as they warmed up, the car would actually lock and pull right. Loosening the adjustment of the cable solved it.
 
Does the car have front discs?
If so was it an original disc car?
Wrong distribution valve will do this....

Ask me how I know..... Lol
 
Before you do anything, make sure the front system works! It should normally be nearly impossible to lock the rears with everything ELSE plumbed correctly and working properly.

Typically the rear gets 75% the line pressure that the front is getting.
Typically the front brakes are bigger and more powerful,than the rear.
It seems to me like the front is not working or your P valve isn't or you are turning the valve the wrong way, or it's not plumbed right.

The fountain is telling you that the fluid is returning.
Disconnect the park brake cables from the equalizer bar. Pop the drums off. Manually push the park brake apply arms to the back, pulling the cables with them. Replace the drums and re-adjust the starwheels, lighly jamming the works up until you can just barely turn the drums. Pump the brake a couple of times. Then jam the shoes some more, until you can just barely turn the drums.Then pump the brakes again. Now back off the adjusters about 3 strokes, or maybe 10 clicks. Tie up the park cables, put it all together and roadtest it again.

Now here is the why of it
Drum brakes are self energizing, that is to say, that when you depress the brake pedal, hydraulic fluid sends the motion out to the wheel cylinders which dutifully push the shoes out to the drums. The drums catch the shoes and wind them up like a big spring and jam it all together to the point that you might want to lift your foot a little cuz it's slowing the car down faster than you anticipated. Now if the rears are adjusted sloppy loose,then the amount of pedal travel you are typically using, may never push the rear shoes into the drums and so you travel many thousands of miles with never having had any rear brakes at all! Now, today you fix all that and adjust the shoes up tight and, lo and behold the rear brakes work again! Maybe better than the fronts.This is bad.

Now after everything is working again, then you can hook up the park brake again, assuming they're not seized!
 
AJ/FormS said:
Before you do anything, make sure the front system works! It should normally be nearly impossible to lock the rears with everything ELSE plumbed correctly and working properly.
Click to expand...

he mentioned having a 318 with a 8 3/4 rear end and 10 inch drums
if that rear end is not original to the car, the brake bias might be off
add to that the possibility of larger, wider rear tires (with a much greater contact patch) and it may very well be possible the rears lock up before the fronts do

(thats the way my 71 with 9 inch fronts acted when i swapped in an 8 3/4 with 10 inch drums, at least untill i upgraded the fronts)
 
One thing to check is grooves on the pads of the backing plate where the shoes "slide" on. I have seen the shoes get stuck in the grooves causing "drag", not necessarily "lock up" In the case I describe, the shoes will eventually set hot/smoke/stink while driving.The wheel doesnt actually "lock up". The fix is, new backing plate, or "polish" the groove flat with a grinder. Let us know what your fix is.
 
Loosen the rear brake adjuster about 5 spoons or 20 clicks. Then, open the hood and Cover every thing with paint on it,including the fenders and cowl, surrounding the m/c. Get a helper to pump up the brakes rapidly until the pedal gets hard and hold it. Now pop the cover off the m/c, and raise it just far enough to see the liquid. Have your helper slowly release the pedal. There should be a huge fountain in the reservoir as the rear return springs force the pistons back into the wheel cylinders. I have seen this fountain have so much energy as to reach the hood above it. So you're in charge of not letting that happen!
No fountain is a serious problem.
 
Like A/J said above, I would think front drums are not adjusted correctly. Rear drums are braking before front.
 
Did you do anything to the front brakes when you did this work on the rear brakes?
Take apart, replace any pieces, adjust, etc.?
It sounds like the front brakes are not doing their share of the braking.
I would make sure all 4 corners are fully bled before doing any further parts swapping.
 
Sorry Op
I see we've been down that road already.
Really, there are only three reasons that the rears will lock up before the front;
1) bad design, and 2) the fronts are not working properly, or 3) there is air inside the M/C between the front and the rear pistons.

as to 1) Well it's a factory Mopar so it ain't a poor design,lol.
as to 2) The rears are locking when braking firmly so we know they're working, right? So,
I gotta wonder if you put the P-valve in the right line.
And if you did install it correctly, then I'm going with the fronts ain't working, or not working right .So you gotta jack it up all 4 wheels off the ground. Then have a helper apply the brakes until the rears are dragging heavily, then go check what's happening at the front.
as to 3) If you had this situation, you'd have a lotta pedal travel, and it wouldn't get hard until she hit bottom, and it would not feel normal.

Now just in case you don't know;
as to the two lines that come out of the m/c; the frontmost line, nearest the rad goes to the rear brakes. It is normally operated by hydraulic pressure. But when the front-brake system leaks or has air in it, then the rearmost piston travels forward until it hits the frontmost piston and operates the rear brake system mechanically. Because the m/c is normally uphill at the front end, air in that inter-piston chamber gets trapped in there. When that happens, there is a gusher in the m/c when you release the pedal as the compressed air slams the fluid out. There are only two ways to get that air outta there; 1) remove the m/c and bench bleed it, or 2) with the brakes gently applied, loosen the frontmost brakeline on the m/c and let it out. It shouldn't take more than two pumps to get it all out, and you have to close the line before the pedal goes back up! Remember to protect your paint cuz most brake fluids eat paint.
That's all I got man, hope something works for you.
 
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