Brake distribution block / differential valve. - Vintage Mustang Forums
 2Likes
  • 1 Post By JRFox
  • 1 Post By Paul1958
 
Thread Tools
post #1 of 8 (permalink) Old 04-15-2018, 07:49 PM Thread Starter
Senior Member
 
Join Date: May 2012
Location: Surrey, United Kingdom
Posts: 364
Brake distribution block / differential valve.

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.
JRFox is offline  
Sponsored Links
Advertisement
 
post #2 of 8 (permalink) Old 04-15-2018, 08:04 PM Thread Starter
Senior Member
 
Join Date: May 2012
Location: Surrey, United Kingdom
Posts: 364
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?
dm289 likes this.
JRFox is offline  
post #3 of 8 (permalink) Old 04-15-2018, 08:13 PM
Senior Member
 
Paul1958's Avatar
 
Join Date: Nov 2010
Location: E. MA
Posts: 5,737
Quote:
Originally Posted by JRFox View Post
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
dm289 likes this.

Okay. Engine Stop. ACA out of Detent. Mode Control, both Auto. Descent Engine Command Override, Off. Engine Arm, Off. 413 is in.
-Buzz Aldrin, Apollo 11 LMP
Actual first words from the surface of the moon

Early Fox: 5.0L HO SEFI conversion, 5-speed, 8.8 w/3.27:1 TL, full dual exhaust

1968 coupe - Ordered 12.15.67 Delivered 2.1.68: 8T01T, 65C, 289 conversion, hanger queen

Last edited by Paul1958; 04-15-2018 at 08:17 PM.
Paul1958 is offline  
 
post #4 of 8 (permalink) Old 04-15-2018, 09:49 PM
Senior Member
 
Join Date: Mar 2001
Location: Tunkhannock PA
Posts: 19,771
Basically you donít get a lot of fluid flow through the block on either circuit because you canít 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 doesnít 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

Tom

One thing great about getting older. A life in prison sentence is less of a deterrence

Huskinhano is offline  
post #5 of 8 (permalink) Old 04-15-2018, 11:09 PM
Senior Member
 
mildensteve's Avatar
 
Join Date: Jan 2018
Location: Keller, Texas
Posts: 382
Garage
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.

mildensteve is offline  
post #6 of 8 (permalink) Old 04-16-2018, 05:35 AM Thread Starter
Senior Member
 
Join Date: May 2012
Location: Surrey, United Kingdom
Posts: 364
Quote:
Originally Posted by mildensteve View Post
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.
JRFox is offline  
post #7 of 8 (permalink) Old 04-16-2018, 06:39 AM
Senior Member
 
22GT's Avatar
 
Join Date: Jun 2009
Location: Southeastern Pennsylvania
Posts: 33,793
I think you need to look inside the plastic reservoir. It is typically divided inside.

Amateur restorer. Well, sometimes I have been paid for it. But not right now.
22GT is offline  
post #8 of 8 (permalink) Old 04-16-2018, 07:26 AM
Senior Member
 
awhtx's Avatar
 
Join Date: Sep 2011
Location: The Hill Country of Central Texas
Posts: 9,141
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.
Attached Images
File Type: jpg Plastic MC Reservoir.jpg (14.8 KB, 5 views)
awhtx is offline  
Sponsored Links
Advertisement
 
Reply

Quick Reply
Message:
Options

Register Now



In order to be able to post messages on the Vintage Mustang Forums forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.

Password:


Confirm Password:
Email Address
Please enter a valid email address for yourself.

Email Address:
OR

Log-in










Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page



Posting Rules  
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On
Trackbacks are On
Pingbacks are On
Refbacks are Off

 
For the best viewing experience please update your browser to Google Chrome