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Rear brakes locking
https://www.slantsix.org/forum/viewtopic.php?t=20517		 Page 1 of 2
Author:  Bren67Cuda904 [ Fri Oct 27, 2006 10:06 am ]
Post subject:  Rear brakes locking
I am about to pull my hair out with these brakes. Car isand locking the rears. 4 drums by the way. Nearly every thing has been replaced. New drums, shoes, hardware, rebuilt wheel cylinders, hoses, front wheel bearing and seals. If I were to stand on the brakes at 60 MPH the rear would lock instantly and I would spin out of control and T-bone myself into the rear bumber of the car ahead of me. This is not an agageration.
Took the car to a shop that also had some older cars in the parking lot. He checked my work and found everything put together correctly. He adjusted the brakes and found it to be very close. Test drive found it still locking. Moderate petal presure from 10 MPH and up will lock the rears. If I were to drive it this way, I would have to leave a 5 sec gap between the car ahead at 60 MPH. If not, this car would be wrecked soon.
The shop thinks it "might" be the perportioning valve. They don't think it is the MC because the petal feels good. They also said I should get an adjustable valve in case I plan to put disk brakes on some day.
I have never seen a perportioning valve be bad.
I told them many times, "maybe theres air in the fronts". They said the petal would feel diferent.
What do you think?
To top of the fun I am having, the water pump is driping out the weep hole. I wanted to make it to a couple more shows before I pulled the motor of the new one.
Brennan
Author:  sick6 [ Fri Oct 27, 2006 10:36 am ]
Post subject: 
could be the proportioning valve, I have seen them go bad.

have you checked the parking brake too? mine used to hang up in my duster.

if you can get the car up on cinder blocks and spin the wheels fast enough you might be able to see whats going on. if its happening at 10 MPH you have a better chance of seeing it up on blocks.
Author:  SlantSixDan [ Fri Oct 27, 2006 11:26 am ]
Post subject: 
Are these 9" or 10" brakes on your car?

Any pedal pulsation under light brake application?

Good-quality, name-brand shoes?

Are you 100% sure no grease, oil or other contaminant got on the shoes during or after installation? Have you checked for leaking rear axle bearing seals?

Did you do the rebuilds yourself on the rear wheel cylinders, or did you buy rebuilts? If you bought rebuilts, they could be causing the problem.

What is the status of your rear brake flex hose?

Drum brake systems do not have a proportioning valve, they just have a splitter block. Disc brake systems have a prop valve.

You do not need an adjustable prop valve when you switch to discs—you just need to swap in the smaller-bore (13/16") rear wheel cylinders.
Author:  Bren67Cuda904 [ Fri Oct 27, 2006 12:19 pm ]
Post subject: 
Bought the shoes at Napa. Told them I want the good stuff. I rebuilt all the wheel cylinders myself except the passengerside rear which I replaced. The new and old one looked the same. These are 9" drums x4. No pulsation. New rear hose. No grease or oil leaks. Replace front shoes again because of possiable contamination with the shipping oil on the new drums.
One guy said maybe the distrubution block's warning switch is stuck, however there is no light on the dash and its plugged in. This seems unlikely to me.
What size wheel cylinders do I need and what size are the disk brake ones. If mine (one side) are to small I suppose the rears would engage quicker and then lock.
My hands are in the air at this point.
I am thinking about getting RMS chassis components Alter-k-tion front disk system and be done with the drums and suspenion. But its $3495.
Author:  emsvitil [ Fri Oct 27, 2006 12:34 pm ]
Post subject: 
Sizes (from 65 manual, but should be same)

Front
shoes 9 x 2.5
cyl 1"

Rear
shoes 9 x 2
cyl 13/16

Master
cyl 1
Author:  SlantSixDan [ Fri Oct 27, 2006 12:52 pm ]
Post subject: 
Spending $3500 for a brakes-and-suspension system is, you'll forgive me, a really silly idea.
Author:  emsvitil [ Fri Oct 27, 2006 12:54 pm ]
Post subject: 
BTW, smaller wheel cylinders would have less pressure applied to the shoes and it would be less likely to lock.


What's the contact pattern on the shoes? I've seen strange things happen up until the shoes are making contact on a large portion of the shoes. With new shoes, they'll be making point contact for awhile.


A test (don't do for very long) for adjustment is to slightly pull the emergency brake so that the rears are dragging then see what happens
Author:  SlantSixDan [ Fri Oct 27, 2006 1:10 pm ]
Post subject: 
Quote:
BTW, smaller wheel cylinders would have less pressure applied to the shoes and it would be less likely to lock.
Quite right. The factory installed 15/16" rear wheel cylinders on almost all passenger car 10", 11" and 12" rear drum brakes from '62 onward. Notable exception was cop cars which got 13/16" rear wheel cylinders to avoid rear lockup under "cop duty" hard braking. Those same 13/16" rear wheel cylinders completely solve the tendency of disc-brake A-bodies to lock up the rears, in much more satisfactory fashion than an adjustable prop valve.

But, there are no smaller-bore cylinders for the 9" brakes.
Author:  Dart270 [ Fri Oct 27, 2006 2:42 pm ]
Post subject:  My 2 cents
Is it possible that the front and rear shoes have different lining material? The material can make a huge difference on shoe "bite" which will affect locking quite a bit. It could be that even if the front and rear shoes are the same material, they are different enough from the stock material that the proportioning is off.

The first thing to try would be an adjustable prop valve in the rear line, which is $45 and a couple of hours work or your shop can install it. You could also try smaller rear wheel cylinders by 1/16" or 1/8" in diameter, as others have suggested. Are the cylinders really specific to 9" drums, Dan?

If it were my car, I would swap on a set of discs on the front. SSBC has some for small bolt pattern if you want to keep your SBP wheels. The nice bang for your buck setup is the 11.75" mopar rotors on 73-76 spindles and upper control arms. Use either the factory calipers or get the slick Wilwood calipers. See the moparaction.com site for a tech article and parts list.

I ran 9" drums from 1988-2000 on my cars. AS SOON as I switched one car to discs, the other one felt like the brakes were completely inadequate and I switched the other car over immediately. One of the best upgrades I've ever done on a car. You can do the factory setup for around $800 with new parts, or about $1200 with the Wilwood calipers.

Lou
Author:  SlantSixDan [ Fri Oct 27, 2006 2:53 pm ]
Post subject:  Re: My 2 cents
Quote:
The first thing to try would be an adjustable prop valve in the rear line
Not me. It might stop or reduce the locking, but I regard that as putting a band-aid over the symptom, rather than fixing the problem.
Quote:
You could also try smaller rear wheel cylinders by 1/16" or 1/8" in diameter
Not available for the 9" drums.
Quote:
Are the cylinders really specific to 9" drums, Dan?
Do you suppose I have some reason to lie...? :shock:
Author:  Charrlie_S [ Fri Oct 27, 2006 3:25 pm ]
Post subject: 
Are you sure the shoes are installed properly? Primary (short) shoe to the front and secondary (long) shoe to the rear. Not being a smart a$$, but I have even done it myself, when in a hurry. Got a set of shoes one time, that had all secondary shoes in the box.
Author:  Bren67Cuda904 [ Fri Oct 27, 2006 5:39 pm ]
Post subject: 
shoes are on correctly. We checked again to be sure.
Author:  Bren67Cuda904 [ Fri Oct 27, 2006 6:17 pm ]
Post subject: 
MP Master Power Brakes has a complete kit including power brake upgrade for $1300 with the shipping. I think I remember someone saying there parts are junk. Do I remember correctly. I need to check the link above for the parts list.
Author:  Joshie225 [ Fri Oct 27, 2006 7:57 pm ]
Post subject: 
You never answered if the front and rear shoes were the same grade of lining material. Some of the "good" shoes are very hard and don't bite for beans. You might have hard front shoes and soft, grabby rear shoes.

If you want disc brakes then get disc brakes. Get some A-body disc brake spindles and update the car.
Author:  Bren67Cuda904 [ Fri Oct 27, 2006 8:03 pm ]
Post subject: 
shoes were the same brand.
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