darth_linux
Senior Member
Nope. I figured it wouldn’t get changed either.
brake booster rebuilt, now brakes are WAY TOO sensitive!
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Sixpactogo
Well-Known Member
Glad you got it solved. Sometimes that is what you gotta do. Sit back and stare at it for a while.
Wayne the RamManInc has put out some absolutely "passionate" video's about boosters and explaining the brake system in general. But I haven't seen any explanation as to what happens if the push rod is not adjusted correctly - by a lot or even just a little bit.
Is it that you don't want the push rod to in any way engage the MBC piston when everything is fully bolted up? That some small, perhaps arbitray gap exists? (ie 1/16 or 1/8 either would work, you just might have a little bit more pedal travel with one vs the other) ?
Something else I find interesting and in need of explaining - why inside a dual-chamber MBC the piston is not solid? That when the front piston moves, the rear piston also moves because it's one solid object. Why is there a spring between the first and second pistons? You are delaying rear brake action when you have that, and I thought it was important that the rear brakes engage not just at the same time as the fronts, but even a little before.
Is it that you don't want the push rod to in any way engage the MBC piston when everything is fully bolted up? That some small, perhaps arbitray gap exists? (ie 1/16 or 1/8 either would work, you just might have a little bit more pedal travel with one vs the other) ?
Something else I find interesting and in need of explaining - why inside a dual-chamber MBC the piston is not solid? That when the front piston moves, the rear piston also moves because it's one solid object. Why is there a spring between the first and second pistons? You are delaying rear brake action when you have that, and I thought it was important that the rear brakes engage not just at the same time as the fronts, but even a little before.
live4theking
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Did this also fix your pedal not returning all the way also?
darth_linux
Senior Member
Yes it did
Gerald Morris
Senior Member
Precisely as you stated, to delay the smaller rear cylinders in order to PREVENT LOCKING THEM UP AND TURNING THE CAR CATASTROPHICALLY! There now exist after market rear brake cylinders designed to prevent lockup. Most of the car's momentum is up front, where the weight of the engine sits, and braking force does best when applied there first, with the rear coming in to drag the car into line only an instant later.
I get contradictory information from the AI search bots when asking questions about the operation or design goals of front and rear brakes.
One answer was this:
"Cars are designed to activate front brakes first and apply more force to them because weight transfers to the front during braking, increasing their grip and stopping power. Rear brakes activate after the front brakes to supplement stopping power and prevent nose-diving by distributing braking force dynamically"
When my prompt was this: "explain rear drum brake hold-off valve" the answer was:
"A rear drum brake hold-off (or metering) valve delays the application of front disc brakes in a disc/drum system to ensure the rear drum brakes engage first, preventing the vehicle from "nose-diving". It holds off the initial line pressure, allowing rear shoes to overcome spring tension and make contact with the drums, and then releases the pressure to the front discs, balancing the braking system. This valve is a crucial component of the combination valve in vehicles with front disc and rear drum brakes."
There's some confusion here. If you don't want the front end to nose-dive, then you certainly can't delay the action of the rear brakes vs the front brakes. Yes? Maybe you even want the rear brakes to activate first ?
Was / is nose-diving a problem in 4-wheel drum cars?
My understanding of the evolution of the brake system from 1967 (4-wheel drums) to 1972 (front disk / rear drums) is that with the first appearance of front disks, a hold-off valve was added TO THE REAR LINE AFTER THE DISTRIBUTION BLOCK as a separate device, the purpose being to throttle (to hold-off) the pressure to the rear drums when the pressure exceeded 250 PSI. If main pressure was 500 PSI, the rears would eventually get 500 but at some controlled rate, they wouldn't get 500 as fast as the fronts got it.
This rear hold-off valve was incorporated maybe in 1969 into the distribution block. The proportioning valve for the front disks came next, it was added a year or so later, when the distribution block became cast-iron instead of brass.
Unless I'm getting this mixed up, and the proportioning valve replaced the hold-off valve and hence like the hold-off valve it also only operates on the rear drums, but it operates differently than the hold-off valve, in which case this needs explaining.
AND EXPLAIN THIS:
Why didn't the master brake cylinder have 2 different size pistons !!! One for the front, one for the rear. That would automatically give a solution for generating different forces for the front disks and rear drums. No need for a complicated distribution block.
I still want to know why there are springs between the first and second pistons in the MBC. Should be a solid shaft. God knows what effect the spring has on rear brake pressure and activation pattern. What were they thinking here?
One answer was this:
"Cars are designed to activate front brakes first and apply more force to them because weight transfers to the front during braking, increasing their grip and stopping power. Rear brakes activate after the front brakes to supplement stopping power and prevent nose-diving by distributing braking force dynamically"
When my prompt was this: "explain rear drum brake hold-off valve" the answer was:
"A rear drum brake hold-off (or metering) valve delays the application of front disc brakes in a disc/drum system to ensure the rear drum brakes engage first, preventing the vehicle from "nose-diving". It holds off the initial line pressure, allowing rear shoes to overcome spring tension and make contact with the drums, and then releases the pressure to the front discs, balancing the braking system. This valve is a crucial component of the combination valve in vehicles with front disc and rear drum brakes."
There's some confusion here. If you don't want the front end to nose-dive, then you certainly can't delay the action of the rear brakes vs the front brakes. Yes? Maybe you even want the rear brakes to activate first ?
Was / is nose-diving a problem in 4-wheel drum cars?
My understanding of the evolution of the brake system from 1967 (4-wheel drums) to 1972 (front disk / rear drums) is that with the first appearance of front disks, a hold-off valve was added TO THE REAR LINE AFTER THE DISTRIBUTION BLOCK as a separate device, the purpose being to throttle (to hold-off) the pressure to the rear drums when the pressure exceeded 250 PSI. If main pressure was 500 PSI, the rears would eventually get 500 but at some controlled rate, they wouldn't get 500 as fast as the fronts got it.
This rear hold-off valve was incorporated maybe in 1969 into the distribution block. The proportioning valve for the front disks came next, it was added a year or so later, when the distribution block became cast-iron instead of brass.
Unless I'm getting this mixed up, and the proportioning valve replaced the hold-off valve and hence like the hold-off valve it also only operates on the rear drums, but it operates differently than the hold-off valve, in which case this needs explaining.
AND EXPLAIN THIS:
Why didn't the master brake cylinder have 2 different size pistons !!! One for the front, one for the rear. That would automatically give a solution for generating different forces for the front disks and rear drums. No need for a complicated distribution block.
I still want to know why there are springs between the first and second pistons in the MBC. Should be a solid shaft. God knows what effect the spring has on rear brake pressure and activation pattern. What were they thinking here?
Gerald Morris
Senior Member
You KNOW that "AI" stands for "Artificial Idiocy." Anyway, let's do some Newtonian Mechanics here:
No, the weight is already there. Granted, if the car smacks into a pillar, the weight then will move further forward, relative to the decreased longitudinal axis of the car I reckon.
Sales gobbldeegook. It's exactly as I stated. One wants some rear drag to keep the damned car from flipping around its center of mass.
A LITTLE, but nothing much, and as you know, I drive ALL DRUM CARS. I suppose if I ****** something up on purpose, I could get one to "nosedive" from the greater mass up front to start with, by maladjusting my brakes, but I don't have enough $ or multiple lifetimes to screw around with this **** for academic purposes. With good suspension and front end alignment, brake adjustment et al... mine don't dive much, and I have hard braked a few times to prevent scratches on my paint as well as interior gore. My old '68 Disgustang was a bit more apt to this, due in part to greater maldistribution of mass up front to start with, AND COIL SPRINGS!!!
DAMN THESE ARROGANT WORD PROCESSOR "AI"s and the FLESH AND BLOOD IDIOTS WITH ENGLISH AS THEIR SECOND OR THIRD LANGUAGE TELLING ME WHAT'S PROPER IN MY MOTHERTONGUE!!! **** THEM! WITH BARBED WIRE.....
Anyway.....
No such hold off valve exists on our '68 ragtop, but its still pure-dee drums....
Different piston diameters would complicate things a bit, but I suspect the simple, elegant method they chose works fairly well. Complicating the MC would make it more FAILURE PRONE!!! Better BY FAR to use more passive, static methods to distribute hydraulic pressure, which then translates back to braking force depending on the area of the piston it's applied to. It works simply and well, sans complication.
Trust a lazy old junky like me: when I admire a system for simplicity and elegance, its because it really does get the job done with the least work.
No, the weight is already there. Granted, if the car smacks into a pillar, the weight then will move further forward, relative to the decreased longitudinal axis of the car I reckon.
Sales gobbldeegook. It's exactly as I stated. One wants some rear drag to keep the damned car from flipping around its center of mass.
A LITTLE, but nothing much, and as you know, I drive ALL DRUM CARS. I suppose if I ****** something up on purpose, I could get one to "nosedive" from the greater mass up front to start with, by maladjusting my brakes, but I don't have enough $ or multiple lifetimes to screw around with this **** for academic purposes. With good suspension and front end alignment, brake adjustment et al... mine don't dive much, and I have hard braked a few times to prevent scratches on my paint as well as interior gore. My old '68 Disgustang was a bit more apt to this, due in part to greater maldistribution of mass up front to start with, AND COIL SPRINGS!!!
DAMN THESE ARROGANT WORD PROCESSOR "AI"s and the FLESH AND BLOOD IDIOTS WITH ENGLISH AS THEIR SECOND OR THIRD LANGUAGE TELLING ME WHAT'S PROPER IN MY MOTHERTONGUE!!! **** THEM! WITH BARBED WIRE.....
Anyway.....
No such hold off valve exists on our '68 ragtop, but its still pure-dee drums....
Well, they DID start making them with different volumes as disc brakes became standard. My trusty Bendix dual pot is still fairly even, but even it has a little front volumetric bias, for an ALL DRUM system. The basic fact remains, most of the mass is up front, hence, the momentum, which is linear and conservative.
Different piston diameters would complicate things a bit, but I suspect the simple, elegant method they chose works fairly well. Complicating the MC would make it more FAILURE PRONE!!! Better BY FAR to use more passive, static methods to distribute hydraulic pressure, which then translates back to braking force depending on the area of the piston it's applied to. It works simply and well, sans complication.
Trust a lazy old junky like me: when I admire a system for simplicity and elegance, its because it really does get the job done with the least work.
Sixpactogo
Well-Known Member
From what I understand about these brake systems, the distribution block used in Drum / Drum brakes might delay the line pressure a bit but its main job was to not lock the rear brakes before the front brakes locked up. The difference between the distribution block and the Proportioning valve is that Disc brakes require about twice the PSI as do drums. That is why the manufacturers came up with the dual diaphragm booster to double the pressure to the front discs. They also needed the Proportioning valve which reduced the pressure to the rear brakes down to about half of the pressure.
Simple hydraulics. You have to balance the two systems.
It would be easier to explain this with paper and pencil, but let's try.
First..... Just look at this as two hydraulic systems and forget this is about brakes. That will remove thoughts about applying rear brakes first or other noise. Then let's also say these are identical systems to remove even more thoughts about brakes.
So.. We've got two identical systems with independent slave cylinders being pushed by two master cylinders, one in the front and one in the rear. There's a twist and they use one actuator (your foot against the pedal). If I was setting something like this up, I would use something to balance the force from applied by your foot to the master cylinders. This is done to compensate for differences in each system, like volume of fluid, spring pressure, friction, wear, etc.
The simple way would be to set up a bar that pivots like this with the two master cylinders on each end of the "see-saw" and your foot pressure in the center.
Since that's not the way that Chrysler et al decided to do that, they added a spring between the two master cylinders instead. So, think of the spring as the center pivoting bar in the above see-saws. Let's say you are applying 100lbs to the pedal and the systems are like the left pic where the pressure is split easily. The spring in the master doesn't come into play like the see-saw is horizontal. Now let's say one system needs to push 25lbs and the other needs to push 75lbs. You have the pic on the right compensating for the difference. The spring in the master allows one piston to move more to compensate.
I've simplified this a bit, but remember that you are dealing with different size shoes or in the case of disc brakes, different size pistons so that more force is applied to the discs. In an ideal world, the front and rear systems would be matched by sizing the piston so the force and volume needed would match. It's not an ideal world though and you need to have something that allows for variance.
Pain meds are kicking in, so I'll stop explaining what I learned sitting in night school hydraulics class so many years before the internet and AI.
Speaking of AI.... Stop... Just stop.
But you're drawing those circuits in parallel. The pistons are in series. The pedal is directly acting on the front circuit, but force is only conveyed by a spring to the rear circuit. Any idea how stiff that spring is - or what happens if it ever breaks? On a 4-wheel drum system, where the wheel cylinder diameters and shoe-widths are already modified front vs rear, why is the spring in the MBC needed?
You will for sure get nose-diving when the spring delays rear brake activation.
You will for sure get nose-diving when the spring delays rear brake activation.
They actually are in parallel. Again, it would be easier to explain with paper and pencil, but they are two separate systems.
This thread reminds me of a cartoon I used to see on cable TV about "cars of the future". (I'm guessing it was from the late 40's/early 50's)
"The latest sure-stop toe-touch hydraulic brakes"
"The latest sure-stop toe-touch hydraulic brakes"
Gerald Morris
Senior Member
THANK YOU Big John, for your excellent explanation by analogy. I for one LIKE the relatively SIMPLE method used by the brake parts engineers to calculate how to optimize braking using naught but logarithmic tables, k = X lbs/in. and such, and I drive the machines so designed with far greater confidence than I do when forced to drive absurdly overcomplicated machines, poorly designed, poorly built, dependent upon extremely faulty and complex systems called "computers." The Martini and Cigar Era produced the Apollo lunar missions after all, and has yet to be equaled.
The spring isn't going to delay anything. It will only start to compress once its rated pressure is exceeded.
Go get a spring out of a ball point pen. Set it vertically on the floor and stand on it. It compresses to the point of where the coils bind.
Now get a coil spring from a dump truck. Again, set it vertically on the floor and stand on it. It doesn't compress because you don't weigh anything close to a dump truck.
And if you do that, please video tape it and post here.
Remove the pivots from my other drawing and add a spring to one leg.
Now just swing the drawing and put them inline. Then add a spring that compresses at 25lbs.
A force of 100 psi is applied to a piston with area 1 square inch, which is connected by a spring to a second piston of area 1 square inch. What is the force experienced by the second piston?
A force of 100 psi is applied to a piston with area 1 square inch, which is connected by a spring to a second piston of area 1 square inch. Does the spring delay the application of force experienced by the second piston?
Force Transmission and Delay
Yes, the spring delays the application of force experienced by the second piston.
(explanation of spring compression, extension, time equilibrium)
A force of 100 psi is applied to a piston with area 1 square inch, which is connected by a spring to a second piston of area 1 square inch. Does the spring delay the application of force experienced by the second piston?
Force Transmission and Delay
Yes, the spring delays the application of force experienced by the second piston.
(explanation of spring compression, extension, time equilibrium)
The additional force needed to drive the disc brake pistons is generated by the size of the piston in the caliper.
Simple hydraulic math from High School Physics.
Take 100 PSI generated by the MC piston. Apply that to a brake cylinder piston that 1.129" in diameter. Do the math (round off) and you end up with a 1 square inch surface on the top of the piston. 100PSI x 1 = 100 pounds of force applied to that piston.
Now take that 100PSI and apply it to a larger 1.596" piston. Again, do the math and we have 2 square inch surface. 100PSI x 2 = 200 lbs of force.
You aren't specifying what force the spring starts to compress. Again, go back to the analogy of pen spring versus dump truck spring.
Any force will compress (to some extent) any spring. Even if I say that the first piston is connected to the second piston by a very weak spring, the answer is the same.
You draw your diagrams as if they are electrical circuits, with current flow, resistance, and voltage drops. That's not how this particular hydraulic circuit works, there is no flow, it's basically static.
To take it another step, the pistons compress, the brake pedal bends etc. Is it enough to have any effect? There's clearance in every pivot point that needs to be taken up. Does it mean anything?
And there is flow. The rear brake cylinder pistons expand outwards, making the volume in the cylinder larger. They expand because the fluid has been forced into the cylinder by the piston in the master cylinder. If it was just increasing static pressure, the brake shoes would be in contact with the drums at all times.
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