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Discussion Starter #1
Hi there,

Can someone clear this up for me about the brake distribution block / differential valve. The basic type found on a drum / drum car.

First I will explain what I know and then I will explain what I am reading contradicting things on.

I more or less understand how the distribution block / differential valve works. The front and rear brake circuits go through this block. The two circuits are separated by a piston valve. In normal operation the piston valve is central. If one of the circuits loses pressure then the piston valve will move off centre towards the side that has low pressure. When the piston valve moves off centre, a plastic switch screwed in the top activates a warning light on the dashboard. I have seen all the pictures and internal diagrams of this.

Okay the bit I have read conflicting information on. When a circuit loses pressure and the piston valve moves off centre, does the circuit that has the low pressure get blocked by this valve?

I have read that yes it does block the leaking circuit to give you protection and still have braking in the other good circuit. So the leaking circuit is no longer being pressurised from the master cylinder and the good circuit is still being worked as normal.

I have also read that no it does not block the leaking circuit. If this was blocked, then the brake pedal would be rock hard as it would be trying to pressurise against a blocked circuit. And if the pedal is rock hard, you would not be able to get any braking action on the other good circuit. And your braking protection is from having a dual master cylinder and nothing to do with the distribution block / differential valve. The only purpose of the distribution block / differential valve is to activate the dashboard warning light.

So the above two paragraphs are not my words. Just a summary of what I have read on various sites.

Any comments on if the leaking circuit is blocked or not by the distribution block / differential valve?

Thanks.

Jeremy.
 

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Discussion Starter #2
Okay I might have answered my own question.

I have looked at an internal diagram of a dual master cylinder. It seems there is a primary and secondary piston that are not directly connected together, but have a spring between them. So pressing the brake pedal would move the primary piston and then either via the spring or with fluid pressure, the secondary piston would also move.

So it looks like the distribution block / differential valve would block the leaking circuit. And the dual master cylinder would still work as it looks like one piston could still work when the other one is facing a blocked circuit.

Am I on the right track or getting this totally wrong?
 

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Okay I might have answered my own question.

I have looked at an internal diagram of a dual master cylinder. It seems there is a primary and secondary piston that are not directly connected together, but have a spring between them. So pressing the brake pedal would move the primary piston and then either via the spring or with fluid pressure, the secondary piston would also move.

So it looks like the distribution block / differential valve would block the leaking circuit. And the dual master cylinder would still work as it looks like one piston could still work when the other one is facing a blocked circuit.

Am I on the right track or getting this totally wrong?


Fluid from one system (front or rear) cannot flow into the other. That is the essence of a 'dual' brake system. But if one side of the system (front or rear) has a leak the differential valve doesnt 'block' the leak.

Paul
 

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Basically you don’t get a lot of fluid flow through the block on either circuit because you can’t compress a fluid and you have a residual check valve to maintain a slight positive pressure of fluid on the wheel cylinders to overcome the return springs on the shoes so the shoes will be in slight contact with the drums cutting down on reaction time. On disc there is nothing to push the pistons back. So the switch doesn’t move or move that much. But when you have a failure on a circuit you now have hydraulic pressure on one side and not on the other and the switch now operates to trigger a warning light
 

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The distribution block sliding valve does NOT block the fluid on the low pressure side. Look at your diagram and you will notice a reduced diameter on the sliding valve which allows fluid flow around it. A properly working sliding valve can be re-centered by properly bleeding the brake system after fixing the cause of the leak/low pressure. The problem is the valves get sticky because of poor maintenance or age. Mine was stuck to one side with the light on even after fixing the brake problem until I figured out how to rebuild the distribution block.
 

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Discussion Starter #6
The distribution block sliding valve does NOT block the fluid on the low pressure side. Look at your diagram and you will notice a reduced diameter on the sliding valve which allows fluid flow around it. A properly working sliding valve can be re-centered by properly bleeding the brake system after fixing the cause of the leak/low pressure. The problem is the valves get sticky because of poor maintenance or age. Mine was stuck to one side with the light on even after fixing the brake problem until I figured out how to rebuild the distribution block.
Thanks for all the great replies above this one, but this one hit the nail on the head.

So the distribution block / differential valve does not block the flow on the failed circuit and merely just activates the dashboard warning light.

And yes looking at the diagram above, you can see the end of the valve as you say is a smaller diameter, thus still allowing fluid to flow.

Great, as this backs up what I read elsewhere.

So another question :)

On our older cars the dual master cylinder has two reservoirs under the single top cover. One for each circuit. On more modern cars they generally have one plastic reservoir that feeds both circuits in the dual master cylinder. So what happens when you have a leak on a more modern system. The fluid in the plastic reservoir would drain out and leave the good circuit starved of fluid. But I guess that would only happen if the person carried on driving the car despite the brake warning light being on? I guess when the warning light comes on, you should safely stop the car and then not drive it again until the problem is fixed. And not to think, well I could probably still be able to drive it home, etc.
 

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I think you need to look inside the plastic reservoir. It is typically divided inside.
 

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There are Master Cylinders and Reservoirs similar to this one that do not have an internal dividing mechanism. If one system develops a leak the large, common fluid chamber will eventually go dry but the "standpipe" above each piston still contains enough fluid to operate that half of the brake system.
 

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