Rear brakes lock up... front disc

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cudajim

cudajim

cudajim
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My 67 Notchback Barracuda has stock Kelsey-Hayes power front disc brakes and rear 10" drums on a 7-1/4" rearend. It stops fine most of the time except in hard braking the rears will lock up and send the car all over the road. It's really dangerous so I have to fix it. I've already tried smaller slave cylinders at the rear; they helped but have not solved the problem entirely. The pedal is firm and at the top. Any suggestions?
 
well you need a adjustable proportioning valve, my duster was doing the same thing so I bought one, didn't install it as I found out my axle was leaking fluid the shoes were coated with gear lube and that was causing my issues, if your not leaking oil onto the shoes than you might need the prop valve.

http://www.summitracing.com/parts/DCC-5249088/?rtype=10
 

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I tried an adjustable proportioning valve on my other car and it didn't make a bit of difference. I am suspecting something is not quite right with the front brakes in that they're not grabbing enough or I have the wrong pads etc.
 
I bought an adjustable proportioning valve off ebay for $25 as I recall. Looks identical except chrome and no MP & Wilwood logos.

I disagree w/ fklskv. Smaller cylinders in the rear would give less braking in the rear.

I also agree that sticky shoes is the most likely culprit, since it didn't lock up previously. Have you checked them? My fairly new 82 Chevy S-10 got grabby in the rear and I found a cylinder leaking on the brake shoes. It was also rusty inside. No telling what GM installed for brake fluid. Imagined an open drum where workers dumped their drinks. I would also bleed all calipers and cylinders. I once couldn't bleed a caliper on a Dodge Aries, even with the bleeder removed. Finally forced rusty gunk thru, then they worked. Nothing but silicone fluid for me since (some FABO's will scream).
 
Mother Mopar has long been noted for their one size fits all proportioning valves that almost always cause the A bodied cars to prematurely lock up the rear brakes under hard braking or less than optimal traction conditions (wet pavement for instance), resulting in the car's tendency to spin like a top.
Making sure the metal rear wheel cylinder brake seal expanders are in place or that you have residual check valve(s) in the rear brake lines, gutting the proportioning valve section of the combination valve, and placing an adjustable one in the line going to the rear brakes, and taking the time to do the testing and adjustment needed to get the settings right after you install it is the usual time tested fix.
 
BillGrissom said:
I bought an adjustable proportioning valve off ebay for $25 as I recall. Looks identical except chrome and no MP & Wilwood logos.

I disagree w/ fklskv. Smaller cylinders in the rear would give less braking in the rear.

I also agree that sticky shoes is the most likely culprit, since it didn't lock up previously. Have you checked them? My fairly new 82 Chevy S-10 got grabby in the rear and I found a cylinder leaking on the brake shoes. It was also rusty inside. No telling what GM installed for brake fluid. Imagined an open drum where workers dumped their drinks. I would also bleed all calipers and cylinders. I once couldn't bleed a caliper on a Dodge Aries, even with the bleeder removed. Finally forced rusty gunk thru, then they worked. Nothing but silicone fluid for me since (some FABO's will scream).
Click to expand...

Bill, all my mopars have always locked up in the rear no matter what I do to them. The cylinders, brakes and all hardware are all new and not leaking at all. Everything bleeds just fine including the new front calipers. I bled them till I was blue in the face. Any other ideas?
 
Smaller dia wheel cylinders will travel a little faster, but with less surface for the hydraulics to push means less pushing power.

fklskv said:
I would think smaller cylinders in the rear would make it worse
Click to expand...
 
Have you changed the rear brake shoes? If so did you get the long and short shoes on the right way. If I remember right the long shoe goes on front and short goes on rear.
 
Right you guys are. Im thinking the master cylinder. The slaves work the opposite. There goes my mistake for the year!
 
I've seen contaminated fluid do this. But it would effect the front as well. Do you have all the return springs hooked up in the right places and not binding? Did you use new hardware and is it correct?
 
It is kinda a dumb idea and clearly not ideal, but you could loosen up the adjustment on the rear drums. If they have to travel a slightly larger distance to apply it might let the fronts do a little more work. Of course if you tap the brakes in reverse they will tighten up again with the self adjusters.
 
Take the drums off and have them machined.They are probably not round on the inside and that would cause the locking up. I have the factory brakes on my car and they work great.
 
Dubob said:
Have you changed the rear brake shoes? If so did you get the long and short shoes on the right way. If I remember right the long shoe goes on front and short goes on rear.
Click to expand...

Wrong, the short shoe goes up front.
 
All good ideas here but I have new drums, new fluid, the shoes are on properly, the springs are all correct and new. Everything is factory setup from a 68 Valiant including master cylinder, brake lines, combination valve, front kelsey-hayes disc brakes etc.
 
Have you replaced the rubber brake line that goes from the metal line to the block on the rear axles? That could be closing up on the inside....
 
Dang, with all those problems you should just drive down the hill and give the car to me! :D j/k

My Challenger did the exact same thing with its 10.8" front disks and 10" rear drums. I went to 11 3/4" disks in the front and 11" rear brakes with no other changes, balanced perfectly now, although I don't recall which wheel cylinders I have on the 11" drums.

You mention that everything is new, have the pads and shoes been on the car long enough to be broken in? Were they installed at the same time?

Otherwise, at this point I would install an adjustable proportioning valve. Not sure why it didn't work for your other car, they should be more than able to balance the braking front to rear when adjusted properly.
 
Most disk/drum cars from the 60s and 70s will lock the rears first, actully a better alternative then the fronts first. Shocks can also play a roll in braking if your getting to much front end dive under hard stops, which lightens the rear.
 
OldmanRick said:
Most disk/drum cars from the 60s and 70s will lock the rears first, actully a better alternative then the fronts first. Shocks can also play a roll in braking if your getting to much front end dive under hard stops, which lightens the rear.
Click to expand...

Yup, they lock first alright, then it sends the car into a dangerous set of loops all over the highway. The shocks are new too as are all the rubber brake lines.
 
twofosho said:
Mother Mopar has long been noted for their one size fits all proportioning valves that almost always cause the A bodied cars to prematurely lock up the rear brakes under hard braking.

Gutting the proportioning valve section of the combination valve, and placing an adjustable one in the line going to the rear brakes, and taking the time to do the testing and adjustment needed to get the settings right after you install it is the usual time tested fix.
Click to expand...

I hate to make brake recommedations, mainly because it's not my a** on the line:). twofosho may be on to something though as far as rendering the factory prop. valve inoperative as to not interfere with a seperate adjustable valve. IIRC a friend with a fox body drag car had a similar issue with that.
 
Seems to me this is:

1 Either a pressure balance problem

or...........

2 a shoe/ pad / problem

You may get to the point where you need to "rig" a pressure gauge, first in one system, then the other to see what you are actually getting. You may need a different prop valve, or smaller rear cylinders.

But don't discount pad/ shoe WEAR IN as well as pad/ shoe LINING MATERIAL. A good example is the two (87/86) Ferd Ranger 2x4's I've had. BOTH had factory rear shoes, and BOTH had know problems where the rear brakes lock up in cold or wet weather. I always had to leave home carefully, and "drag" the brake for about 1/2 a block as well as make a carefull "first couple" of stops to get the linings dried out and warmed up a little. Sometimes this happened again in the afternoon, but generally it's a "once in the morning" thing.

So you might try different types of lining front/ rear.
 
Another thing to check along with the above info is your emergency brake. If the cables are not releasing fully it will shorten the distance from shoe to drum. I have had it happen on 2 cars before. After freeing the cables I didn't have a problem and I could increase the pressure to the back drums.
 
I don't see this as a "problem" - I see it as a known issue with all disk/drum Mopars.

I'm willing to bet that if every one of the members posting on this thread running disk / drum went out and did one emergency stop from cold - 98% would end up sideways.

Give it a whirl folks - you'll be horrified. :)
 
I agree with the emergency brake cable theory. I spent hours trying to figure it out . it seems that the slightest touch will cause the primary shoe to grab the next thing you know you're in full lock up . the same as if you had applied the emergency brake . good luck to you
 
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