Technical Master Cylinder Issue

In my opinion you want the reservoir that gets actuated first to go to the front brakes, there is more weight over those tires than in the back so it will take more stopping force to stop those wheels. If it was the other way around the rear brakes would apply first and likely start locking up trying to slow the moving mass from behind before the front brakes could fully engage.
I wish you would go back and re read my first comment, give it some thought, maybe loosen up your rear brake shoes so you get more pedal travel before it expands out to the drums. See if that has an effect on the front brakes, if it does a little then figure out how to get more, either by cracking the line or putting in a manual proportioning valve in the rear line to reduce the volume going back there.

 

I have been thinking about it...I have a list of items i did not think about to try this weekend...got some parts to gather and I will be getting after it...i need this figured out this week..lol..thanks for the input!

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Those ARE NOT proportioning valves you’re speaking of.
They are residual pressure valves. Completely different

 

Yup totally thinking about the bigger problem and not what I'm typing...

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I’m curious how many people have had pressure gauges on the front and rear of a brake system at the same time.

OEM stuff that has disc-drum systems typically have metering valves and proportioning valves, the metering valve blocks pressure to the front until roughly 30psi to allow the rear to start to apply, because of return spring pressure, keeps the vehicle from nose-diving and unloading the rear, then when the pressure reaches the split point of the proportioning valve it limits pressure rise in the rear to avoid lock up. There is a reason for these valves, the master cylinder does not determine which applies first. But what do I know.


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Obviously more than I at this point

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Those are residual pressure valves. Different thing entirely.



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Have you ever bled brakes?

With one bleeder open, any corner, you get a full stroke.



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You’ll have some brakes, but not much. At least that’s my experience with stock GM and Mopar brakes when something fails.

I had a Buick where the proportioning valve casting was slightly porous, so had to keep adding fluid. If I forgot, brakes would get air in the lines, and wouldn’t work well.

My Dakota has blown a rear brake line (rust) and a front brake hose (also rust). Both times, pedal goes real soft. There is still some braking, but not much.

I’m sure that a dual circuit system is better than a single circuit, but it’s not something I really want to experience again.



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There's a reason why it's called primary. Until primary circuit has resistance/pressure built up will the secondary circuit begin pressurizing when working properly. Why Ford made their larger reservoir (fronts) to the rear of master I'll never understand.

 

That picture is wrong, the primary seals sit between the ports at rest, it takes roughly .030” of movement to cover the vent port and start building pressure in the primary system, then the secondary starts moving, for all intensive purposes instantly.


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Yes the circuit that hasn’t failed will still work, that’s the purpose of the tandem master, there will be lots more pedal travel and remembering the front does most of the braking if you lose them it will feel a lot like no brakes. Part of the reason lots of new stuff is diagonal split. That throws the delay in the master out the window doesn’t it?


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What is the pedal ratio? Is it applying pressure to the front? How much play does the pedal have before it applies pressure to the master cylinder? Just a couple things to look at

 

Ford wasn't the only one. Check out some late '60s-early '70s GM cars, where the primary went to the rears. There are probably other examples from AMC and Chrysler.

 

You sort of answered your own question.
This comes up over and over.
It is not common, and likely not the case in this particular application, but dual pressure masters are available, and have common bore size. Example...Square area of a complete circle being 1” in diameter would not be the same as a circle of 1” diameter with a floating rod of 1/2” diameter moving through it. Think of a doughnut shape vs a complete circle. Square area is completely different with the same bore size.

As far as front to rear location of output ports, the complete designed system needs to be considered. When you apply the pedal on an empty system, you are exerting force against springs. Every design variation has a purpose.
“Generally” the first chamber in a master that moves and starts to compress is the port farthest away from the actuator rod. As the pressure rises (that port applying force) the resistance to compress increases, creating a more solid link and the next port starts to generate pressure. Although the way I describe it sounds like it happens in stages (and it does) it happens almost instantaneously.

 

Except that the pedal directly moves the primary piston, which is the one closest to the mounting flange, creating pressure, which is what moves the secondary. So no way can the one furthest from the actuator rod move first as you stated, unless I misunderstood you.

And yes I understand there are stepped bore masters, as you said not typical.




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Yes, I understand the dual reservoir for redundancy, and the delay with an adjustable, external proportional valve in the rear line, like I have.

So, what does the shuttle valve in the factory proportional valve assy. do? I remember having one of those stick years ago.

 

Mostly turns the light on...


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Back the rear brake shoes adjuster off some to loosen up the rear brake shoes to allow more travel in the wheel cylinders....not knowing the size of the rear wheel cylinders and the master cylinder bore the cylinders could be filled before the front calipers are .. causing the rear system to fully engage the shoes limiting the pedal travel

 

That's exactly what I'm saying only worded differently.. Your "intensive" and my "working properly" are of equal reference. Your point and mine only explains the simple physics when system "begins working". As a whole the system can not have any sort interruptions of any sort, what ever that maybe. Single Circuit Two Circuit it don't matter all whole system will be compromised.
Just to make you happy. Yes photo is very basic.

 

I dunno.. that may result in losing height at the pedal. If anything I will over-adjust them a few clicks and back them off after the bleeding is done.

 

This is what I use, a simple distribution block with a rear bias adjuster. I have the black anodized one, $30 less. No valving internally to fight with, it is really a glorified brass block with the bias adjuster.
https://www.ebay.com/itm/CPP-PROPOR...ON-BLOCK-CHROME-SWITCH-AND-WIRE-/262771374225

 

Ok sorry I misunderstood what you were saying then.


I’m also finding it a good conversation, despite what I feel some may think, my intention isn’t to argue, all of this would be simpler sitting around with a beer and a white board rather than trying to interpret text

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teach us,
what is the difference between a
1'' disc/drum master cylinder and a 1'' disc/disc
and a 1'' drum/drum

oh I see, no one uses the aluminium brake master cylinders even though they fit the 7'' dual can boosters and are 1'' bore, because you cant get the brake unions, as no one knows what they are, as they have different thread forms front rear - maybe non double flare

or is it something to do with dot 4 brake fluid in the later and earlier stuff
or, IF I am using 2004 metric s10 calipers in my econo kit, should I be using a 2004 s10 brake master - disc drum,

 

So I know I haven’t had every type of combination valve in my hands,,, so I’m guessing it’s possible that you’ve not seen some of the ones I’ve seen and I’ve not seen some of what you’ve seen.

I’m telling you 100% for sure and certain that the shuttle valve kicks over and puts a hell of a blockage on the busted line side. It doesn’t seal it off completely closed. If you were closer I’d drive over, we can take the rear bleeders out & go for a drive. Hell let’s pull a loaded trailer too.
When we get back to your place we’re going start drinking and try to bleed the rear brakes without resetting the valve pretending we don’t know it’s there. Long enough to get get 1/2 drunk and We start a thread on the hamb about not being able to outsmart brake fluid.

Then after we’re 1/2 ways drunk we reset the shuttle valve and be done with good pedal in 5 mins.

 

Firstly it depends on who’s the manufacturer and application but generally the master cylinder will have a integral RPV on the drum side. (All wheel cylinders can suck air, every one. Some have a a special cup inside or need an RPV). Some master do some don’t but that’s the difference. If they don’t then there’s no difference other than possibly volume of the reservoir.

Some makes and models have no difference in master between power and manual brakes while others have bore size differences and end cup differences.

So blanket statements are going to not be correct in all cases. You’ve got to be specific and get good information.

 

That’s the beautiful thing about this place, we all have different experiences


Would you happen to have a specific application? I’m interested in doing some research on it.

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The last one and very recently was my DD Chevy truck. It blew a rear wheel cylinder after 13 years of faithful service. It blew, lost the first 2/3 of the pedal and then grabbed. Next attempt at stopping was 1/2 pedal and firm. Stayed that way for a over week a week driving it everyday till I could get to it. Master cylinder reservoir wasn’t empty.

 

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