Rear brakes lock up, front factory disc 67 Barracuda

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cudajim

cudajim

cudajim
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I have a 67 Barracuda notchback 6/auto with stock Kelsey-Hayes disc brakes from a 68 Valiant. I swapped the entire front brake assy., power brake and master cylinder, distribution block and brake lines. The rear is 10" drums with the lightweight rearend. The car stops ok but the rears lockup almost every time I step on the brake. The peddle feels ok (not mushy) so I think it's bled just fine. Any ideas on how to fix this?
 
I've read that most factory Mopar's (be they drum/drum or disc/drum) are bias to the rear. The fix is an adjustable proportioning valve in the rear line to limit (adjust) the line pressure. This is considering you have a healthy brake system with no bad parts. See the attached picture of what you need.

I believe in your case there should be two factory valves, a second one to the rear. Did you install that as well?
 

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dano said:
I believe in your case there should be two factory valves, a second one to the rear. Did you install that as well?
Click to expand...

Yes, there is a second valve on the line to the rear brakes on earlier cars. I'm not sure where the cutoff was. 68 cuda has it, but by 73 it was all in one. If you cannot get one, the fix is described above by dano.
 
66fs said:
Yes, there is a second valve on the line to the rear brakes on earlier cars. I'm not sure where the cutoff was. 68 cuda has it, but by 73 it was all in one. If you cannot get one, the fix is described above by dano.
Click to expand...

So what's the 2nd valve do?
 
There was no holdoff (delay) valve used in A-bodies until '75. The two valves used in the '67 A-body would be the residual-pressure valve and the proportioning valve. The RP valve is located behind the tube seat in the front port of the master cylinder, which feeds the rear (drum) brakes (prior to '67, the RP valve was a separate external unit); its job is to keep a slight amount of pressure in the line when the brakes are released so air won't be sucked into the system past the rear wheel cylinder seals. The need for the RP valve was eliminated in '73 with redesigned rear cylinder seals, and at this late date most drum brake wheel cylinders have the redesigned seals so an RP valve is nowhere near as necessary as it once was. An RP valve must not be present in any port of any master cylinder that feeds disc brakes, because discs will drag if there is any residual pressure in the line.

The proportioning valve is part of the brass combination splitter block/valve assembly you say you swapped from the '68 car. Its job is to reduce pressure to the rear drums because the drums will want to lock up at the line pressure levels required to get effective braking out of the discs up front.

Disc-brake A-bodies did have an endemic tendency to lock the rears prematurely. Everyone griped about it in road tests of the time. Consumer Reports wailed about it. Chrysler did nothing about it, which is dumb because the zero-dollar fix was right on their shelves, see here (and TSB here and MTSC info here). 10" and 11" drum brake wheel cylinders are interchangeable. I routinely swap the 13/16"-bore rear wheel cylinders onto 10" rear drums on cars with 8¾" rear axles, and the ¾"-bore rear wheel cylinders onto 10" rear drums on cars with 7¼" rear axles, and it fixes the problem all the way fixed without having to jack around with an adjustable proportioning valve or otherwise re-engineer the system.

¾" rear wheel cylinder is NAPA 37863 or Raybestos WC-37863 or Bendix
34076 or Wagner WC123412 or Centric 134.67015 or Chrysler 4423 852.

13/16" rear wheel cylinder is NAPA 37696, Raybestos WC37696, Wagner F113704, ACDelco 18E268, Bendix 34064, Chrysler 4313 493, or Centric 134.67013.

(And while you are in there swapping the wheel cylinders, make sure to check carefully for oil-contaminated shoes, which will grab and lock no matter what wheel cylinders you've got)
 
Had the same issue for awhile in my 75 Swinger. took the drums off and the internals all looked relatively new but there was more brake dust in there than I had seen in awhile, so I blew it all out and it hasn't done it since
 
Slant six- excellent info. I've used a drum brake prop valve and saw no lock up of the rear. This was on a 72 Duster. Not saying it's the right way but it sure seemed to work.
BTW- this was on a big bolt conversion.
 
cudajim said:
So what's the 2nd valve do?
Click to expand...

The "second valve" is on the left hand side and is an additional block in the line running from the "front T" to the rear flex line going to the differential. It is the Proportional valve that limits pressure to 500-540 psi for the rear brakes on the Kelsey-Hayes disc brake "A" bodies. The first valve is inside the master cylinder, for the rear drum brakes only, on disc brake cars. In my first reply I was refering to the "front T" that splits the lines from the master cylinder to each of front wheels and the back wheels. There is no valve in the "front T" from 1965-?. I know the proportioning valve was moved to the "front T" by 1973. Sorry for the confusion...
 
I ordered the Napa 37696 slave cylinders for the rears. Thanks Dan for the info... I'll let you all know how it works out.
 
Alright, so I installed the 37696 (13/16") wheel cylinders onto the rear brake assemblies and it did help considerably but they still do lock up. Now having said that, I looked back at Dans comments and realized I should have used the smaller 3/4" cylinders because I do have the 7-1/4" rear end.... duh. All is not lost though, I'll buy the proper ones today and use the 13/16" slave cylinders on the ragtop Cuda which does have an 8-3/4" rear end.
 
Update... I installed the 13/16" cylinders onto the 67 ragtop cuda which has the 8-3/4" rear-end. It worked great! On this car I have an adjustable proportioning valve and I now get front brakes locking just before the rears. I'll adjust/tweak the valve today. The other car with the 7-1/4 rear isn't bled yet but I'll get on it today.
 
disc cars had a second valve, its often called a delay valve and sometimes a prop valve. this valve is located about 14 inches away from the distribtion block down the frame. its almost under your feet below the floor board. it looks like this pic attached.
 

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I added a proportioning valve that looks like the one Dano shows, other than the Wilwood name and yellow seal, plus mine is chrome. I recall $22 on ebay. Either from the same factory or those Chinese copy great. Usually the former and they run a night shift to make off-label parts.

SlantSixDan gives excellent info, as always, however I would add that for people who are in-tune with their car, an adjustable valve might make sense. There are many variables that can change the braking from the factory setup - different size or make of tires front & rear, different weight distribution from factory, different friction materials (very touchy on drum brakes as Dan relates). The factory proportioning valve was just to get it close enough, though looks like Chrysler messed up.

You don't want to adjust the proportioning valve for every trip (dead body in the trunk?), though braking conditions do change, but you may want to adjust after major design changes (bigger wheels in rear). If a circle-track racer, you probably would fine tune for every race. The downside to an adjustable valve is a potential to leak brake fluid, having something that a clueless mechanic could bugger with, and the temptation to keep tweaking it (some love that).
 
Update... I finally got around to replacing the slave cylinders on the 69 fastback. It's got a 318 and a 7-1/4" rear-end so I used the 7/8" cylinders. I also have an adjustable proportioning valve. The brakes are now much better, meaning the rears aren't nearly as sensitive as before when I had the bigger slave cylinders but they will still lock up on hard braking. It almost seems like the fronts (Kelsey-Hayes) are not grabbing as they should. Any suggestions?
 
cudajim said:
Update... I finally got around to replacing the slave cylinders on the 69 fastback. It's got a 318 and a 7-1/4" rear-end so I used the 7/8" cylinders. I also have an adjustable proportioning valve. The brakes are now much better, meaning the rears aren't nearly as sensitive as before when I had the bigger slave cylinders but they will still lock up on hard braking. It almost seems like the fronts (Kelsey-Hayes) are not grabbing as they should. Any suggestions?
Click to expand...
I thought you were supposed to use the 3/4" cylinders in your application......
 
Thanks Dan, I have the 11X2 1/2 on my 67. They will lock up earlier and althugh they haven't been a big problem I'll change over the wheel cylinders this winter, and fill with new brake fluid. What's the best to use? I have 73 disks up front and 11 X 2 1/2 in the back. The splitter is from the 73 disk car with 10" drums.
 
slantsixdan said:
There was no holdoff (delay) valve used in A-bodies until '75. The two valves used in the '67 A-body would be the residual-pressure valve and the proportioning valve. The RP valve is located behind the tube seat in the front port of the master cylinder, which feeds the rear (drum) brakes (prior to '67, the RP valve was a separate external unit); its job is to keep a slight amount of pressure in the line when the brakes are released so air won't be sucked into the system past the rear wheel cylinder seals. The need for the RP valve was eliminated in '73 with redesigned rear cylinder seals, and at this late date most drum brake wheel cylinders have the redesigned seals so an RP valve is nowhere near as necessary as it once was. An RP valve must not be present in any port of any master cylinder that feeds disc brakes, because discs will drag if there is any residual pressure in the line.

The proportioning valve is part of the brass combination splitter block/valve assembly you say you swapped from the '68 car. Its job is to reduce pressure to the rear drums because the drums will want to lock up at the line pressure levels required to get effective braking out of the discs up front.

Disc-brake A-bodies did have an endemic tendency to lock the rears prematurely. Everyone griped about it in road tests of the time. Consumer Reports wailed about it. Chrysler did nothing about it, which is dumb because the zero-dollar fix was right on their shelves, see here (and TSB here and MTSC info here). 10" and 11" drum brake wheel cylinders are interchangeable. I routinely swap the 13/16"-bore rear wheel cylinders onto 10" rear drums on cars with 8¾" rear axles, and the ¾"-bore rear wheel cylinders onto 10" rear drums on cars with 7¼" rear axles, and it fixes the problem all the way fixed without having to jack around with an adjustable proportioning valve or otherwise re-engineer the system.

¾" rear wheel cylinder is NAPA 37863 or Raybestos WC-37863 or Bendix
34076 or Wagner WC123412 or Centric 134.67015 or Chrysler 4423 852.

13/16" rear wheel cylinder is NAPA 37696, Raybestos WC37696, Wagner F113704, ACDelco 18E268, Bendix 34064, Chrysler 4313 493, or Centric 134.67013.

(And while you are in there swapping the wheel cylinders, make sure to check carefully for oil-contaminated shoes, which will grab and lock no matter what wheel cylinders you've got)
Click to expand...

Great information
340 challconv
 
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