Unbalanced brakes

[French demon]

Hi all
Today I was driving my Demon on a countryroad when after a turn, I saw a traffic slowdown in front of me. In fact, it wasn't a slowdown, but the cars were fully stopped and very close. I realized tardively and I had to make an emergency stop.
Problem, the rear wheels block and the front ones not.
My speed was approximatively 50mph and I stopped before knocking the car in front of me (with screaming and smoking rear tires).
I would like to know what to do to balance correctly my brakes
I have front disk brakes and 10" drums in rear. Disks and pads ok. The booster and the master cylinder are new, but aftermarket. The proportioning valve seems to be original to the car. I think that the bleeding has been correctly made.
An idea?

 

[CFD244]

@slantsixdan just went over this with me on a residual valve question that I had. His links are in my thread. I suggest you read them Francois, good info....See post #17


WHO'S GOT THE SCOOP ON RESIDUAL VALVES?

 

[moparlvr166]

Possible incorrect size rear wheel cylinders, or you can add an adjustable rear brake proportioning valve to keep the rear brakes from locking up.

 

[Inertia]

I'd be checking to see if the front calipers are seized.
The plastic piston calipers have a chronic problem seizing.
Pry the piston/pads away from disc, if it'll move, then pump to see if it'll re-apply .
Process of elimination, prove caliper not seized, and m/c operation.
Good luck .

 

[ch1ll]

Eherenburg has a bunch of brake stuff on eBay site ricksmopars to fix this problem. Everything from master cylinders to brake cylinders and propotioning valves as well. These are the parts he uses on his cars so you know they are not junk. He also wrote an article within the last couple of years in mopar action about this.

 

[MoparLeo]

Check the simple stuf first. Check the rear brakes. Could be too tight, seepin or leaky wheel cy.linder getting on the brake shoes, leaky rear axle seals as well.
Before you d anything you need a complete visual inspection before you srtart to assume things need to be done.

 

[French demon]

Effectively, I'll try to go to the simple stuff first

Is this a proportional valve and can it be defective?
It's original and is 53 y.o.

 

[MoparLeo]

You should also check therr brake lines. All rubber lines should be repla ed as well as rusty steel loines.
Don't just bleed but flush the brake lines out.
Brake fluid absorbs wtaer and after just a few years rust starts to form in the lines, wheel cylinders, calipers, master cylinder, valves etc. Anything in the hydraulic system.

 

[AJ/FormS]

Is this a proportional valve and can it be defective?
It's original and is 53 y.o.

This does not look like a proportioning valve
It looks like a simple Safety switch.
You need to add a Proportioning valve to it.
Did your dash brake lite come on, the one that indicates to you that the Parking Brake is left on?
If no, does it work?
If yes, we need to figure out why.
Was your pedal high and hard before panic braking?
Is it still high?
Here is what a Mopar P-valve looks like.
The block on the left does the Proportioning
The part on the right is just a distribution block for the front brakes.
The left and right parts are NOT interconnected except by a shuttle valve, sealed at both ends, that grounds the dash lite switch, shown at the top center, if one or the other brake systems has failed.

Drum/drum cars do not get a Proportioning valve., rather just a distribution block, as your car seems to have. The Distribution block looks very similar except for the missing integral block shown on the left.

 

[AJ/FormS]

here is a pic of your all-drum Safety Switch.
In 1970, if a car had factory Kelsey Hays disc brakes up front, it might have had a Metering Block in the front lines. or it might have had a divorced Proportioning valve, downstream, maybe as far as under the front driver's seat. I forget

 

[72bluNblu]

Stock Mopars frequently are rear brake biased. Nothing has to be wrong with the system for the rears to lock up first, it’s probably factory stock.

Changing the master cylinder will have no effect on the front/rear brake bias.

Changing the rear wheel cylinder bore will change the front rear bias, and is an option if you can find a different wheel cylinder bore.

Adding an adjustable proportioning valve is probably an easier way to adjust the bias, because there aren’t a ton of available wheel cylinder bores widely available anymore

Effectively, I'll try to go to the simple stuff first

Is this a proportional valve and can it be defective?
It's original and is 53 y.o.
View attachment 1716317796

That is a factory proportioning valve, you can tell because the line nuts are different diameters.

They can be defective, but again just because the rears locked up first doesn’t mean the prop valve is defective. That was a pretty common situation with factory Mopars, especially the disk/drum cars.

Front/rear bias is also changed if you run a staggered tire set up, or have changed the front/rear ride height. Running with the front low and rear of the car high (fairly common muscle car stance) takes weight off the rear and makes it more likely to lock up the rears first.

 

[Bewy]

The problem can be caused by many things as outlined by others. If everything checks out, you could try brake shoes with a different friction co-efficient.

 

[French demon]

This does not look like a proportioning valve
It looks like a simple Safety switch.
You need to add a Proportioning valve to it.
Did your dash brake lite come on, the one that indicates to you that the Parking Brake is left on?
If no, does it work?
If yes, we need to figure out why.
Was your pedal high and hard before panic braking?
Is it still high?
Here is what a Mopar P-valve looks like.
The block on the left does the Proportioning
The part on the right is just a distribution block for the front brakes.
The left and right parts are NOT interconnected except by a shuttle valve, sealed at both ends, that grounds the dash lite switch, shown at the top center, if one or the other brake systems has failed.

Drum/drum cars do not get a Proportioning valve., rather just a distribution block, as your car seems to have. The Distribution block looks very similar except for the missing integral block shown on the left.
View attachment 1716317874

here is a pic of your all-drum Safety Switch.
In 1970, if a car had factory Kelsey Hays disc brakes up front, it might have had a Metering Block in the front lines. or it might have had a divorced Proportioning valve, downstream, maybe as far as under the front driver's seat. I forget
View attachment 1716317879

Thank you all for your advices.
Now I know all possible options for my problem : new wheel cylinders, an extra prop.valve or simply checking my brakes components.

@AJ/FormS : I have original KH disk brakes and the valve is similar to the first one you posted. It has a side mounting tab. It's not only a distribution block.
A larger view, we can see the mounting tab on the left :

And for answering to your questions, all is normal. Pedal height and firmness, handbrake light. Rubber lines, front pads and rear shoes are only 4 years old. Booster, MC and brake fluid are 6 months old. No stance, identical 14" wheels...
Even the valve warning light works (at least it worked when I was replacing the brake booster). Now, in driving conditions, it no longer lights.
No, everything works normally except an imbalance rear/front.

I think about replacing the original prop.valve by a new one.

Or maybe one including a metering valve like this one :

Or third option : rebuilding my valve.
1970 - 1976 Mopar Combination Brake Valve Seal Kit | Muscle Car Research LLC

Complete valve rebuilding instructions :
1970 - 1976 Mopar Combination Valve Rebuild | Muscle Car Research LLC

Is it a clever idea??