Rear Brakes locking up issue

[mr. fix it]

So I was off to another show n' Shine last night and snooped under every Mopar I could find with drum and dual M/C setup.

Every one of them had either a stock or aftermarket proportioning valve...
A. B, C, & D body had the valve..

I'm ordering one today

Thanks for everyone's input on this.
I'll post an update in a few weeks more than likely since I am heading out of business and vacation for the next week or so.

 

[mr. fix it]

Well, i got a hold of a 67 Polara proportioning valve today thanks to a local Mopar friend. I'm in the midst of cleaning it up and readying it it for installation.

I did some research too and found out that the wheel cylinders play a big part in the proportioning of the rear brakes as well.
If I replaced mine with another year the piston diameter may not have been correct for my car which can cause this issue.
I believe I bought for a 66 model but will have to look up my receipts'

 

[Big_John]

I think what you were looking at was a distribution block with the safety switch.

Here's a cutaway of that block/switch.

As you can see, there's a piston that is balanced between two springs. When one side loses pressure, the piston will push to one side and make contact with the switch.

Here's a cutaway of a proportioning valve. This has the same piston and switch, but there's a regulator spring in the circuit to the rear brakes.

Proportioning valve. Compare it to the above block and you'll see the offset in the body for the proportioning valve on the left off the picture.

 

[Big_John]

Here's a good video explaining the differences. About 4:45 or so, he gets into the safety switch.

 

[cantflip]

Here's a good video explaining the differences. About 4:45 or so, he gets into the safety switch.

I find that guy kinda fun to listen to... and he does have some good information under the "i'm trying to convince you" lingo and tone

 

[mr. fix it]

I became a bit confused on the comment about having the rears engage first then the fronts and the car going into a spin @ the 7:40 mark.

All the cars I have driven if the rear's lock then the spins begin.
The fronts' locking and you push like mad.

I think that I confused what he was telling the viewers or he made a mistake in his comment.

 

[mr. fix it]

And C bodied car never had a metering pressure reducing valve??
11:50+ in the video.. go figure that one.. sounds like I might be telling the truth...

 

[Big_John]

And C bodied car never had a metering pressure reducing valve??
11:50+ in the video.. go figure that one.. sounds like I might be telling the truth...

The C body doesn't have a pressure reducing valve. I believe that is true. They do have a metering valve for the front brakes as he said from 69 on. My 70 300 has a metering valve.

As I understand it, from reading my FSM, the metering valve does exactly what he says. It allows the rear brakes to apply first. Keep in mind that's a disc/drum combo and there is nothing like that for a drum/drum combo.

While I think he's exaggerating just a touch on how it works, I can tell you that it does work very well... My car will stop pretty hard if needed. I owned C-body cars back when you didn't put them away in the winter and the disc brake cars were about as good as it got until ABS came out.

 

[Big_John]

BTW, I think you are on the right track with wheel cylinder size.

 

[mr. fix it]

ok so I have added a stock 67 distribution block and the brakes other than some adjustment issues, the brakes are working much better.

I'll let it go for a week or two before closing the lid on this can of worms. thanks for the input guys!

 

[mr. fix it]

Well a Deer put the brakes to the test last night. I was doing about 40mph when it decided to jump out about 50ft in front of me.
I jabbed the brakes and had no rear wheel lock up

Luckily there was no strike and we both made it home safe and sound!