Unbalanced brakes

AJ/FormS said

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

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AJ/FormS said

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

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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?? :)