Whipped - I have been picking up info here on the brake issue. Also some various other fourms so at best its all people like us theorizing. I sure would like to find something from SSBC or GM white paper though. We have enought sharp minded people here we could write our own!

I ended up acquiring a complete rear end to swap out so I can have the JB6 setup. The backing plates are different and I even looked at drilling and tapping new mounting holes but there is not enough room. It makes it an expensive swap and its not even a guaranteed fix. At the minimum I hope to get it so the self adjusters will work properly. My truck would hop every time I touched the brakes as the rears were out of adjustment and it drove me nuts to the point I started hitting the parking brake with my left foot when I hit the brake pedel!

Now I'm on a quest to help solve the spongy brakes and I think we have it narrowed down to using a differnt MC.

This is kind of like choosing a cam, heads, intake, and torque converter but without the aid of benchtop dyno!!!

TJ

JB5's were only on the 10 bolt. JB6's were availaible on the 10 bolt and 14 bolt. The 14 bolt got JB6's JB7's and JB8's. JB= Vac boost JD= Hydro boost (JD6,7,

TJ

Ok black, I figured an easy way to calculate the increase or decrease in braking power from the MC without having to calculate the area of the pistons.
Same goes for the calipers and rear cylinders comparison.

And before you install your JB6 set-up, I would strongly suggest you try the 34mm MC. Easy, simple install.
Then, if you're still not happy with your brakes, you can go to the JB6



Lets say that you push on the brake pedal with a certain amount of force: Say 100lbs

Just remember that going to a smaller diameter MC piston will give you more braking power but also more pedal travel.
This will not affect your brake distribution much It will a bit because of the MC internal springs.

Same with going to a bigger diameter caliper piston/rear cylinder: it will give you more braking power but also more pedal travel.
This will directly affect your brake distribution!



How much is the difference going from 40 to 34?


Divide diameter B from A and square the result. That will give you the percent increase or decrease.

square (A/B)


So square (40/34)= 1.38 or a 38% power increase! But also 38% more pedal travel. (Or as if you're pushing with 138lbs on the 40mm MC)


Now going to the 3" calipers instead of the 2.5": 44% increase power to the front!!

You'll end up with a brake bias way to the front!!


Same with the 1 3/16" rear cylinders instead of the 1": 41% to the rear!! Regardless of the brake distribution valve!

1 1/16 would be nice... But there is a 1 1/8" model for a 27% increase.... Maybe too much...


I think using the GMT800 MC is a great upgrade because the extra volume going to the back takes care of the slack that the auto adjuster doesn't take.



Last edited by CrazyHoe on Fri Aug 03, 2012 9:47 am; edited 2 times in total

Perfect, Thanks!

I see why your saying to do the MC first. Going to that 34mm piston size will provide more force but more pedel travel as well. The JB6's have the 1 3/16 cylindars so that will add more pedel travel to the system as opposed to the JB5's having the 1".

Now I am laying things out and can do different scenarios, so I think...
the GMT800 Hydro is 37mm so that would provide marginal 17% increase. However it gets rid of the split setup and relies on the prop valve to do the distro. This seems like the best first step to compare apples to apples. Then if I swapped in the JB6 but used the 1 inch wheel cylindar instead of the 1 3/16 I get a 41% increase directly to the rears. If that were too little pedel travel then the 34mm MC would add some back in.

Is my thinking in line?

I think I will just need to dive in and start playing these "LEGO's". Here's the kicker I have never messed with brakes, bled a MC or changed pads for that matter. I will be learing on multiple fronts!

Thanks,
TJ

The thing is though, the 40mm MC should have less pedal travel but there's something wrong about it so going to the 34mm might give you less travel and a firmer pedal.

Hey!!!! If the JB6 uses the 1 3/16 cylinders, that could be where they get their power from... Besides the fact that they auto adjust...

98Blackss wrote:

Then if I swapped in the JB6 but used the 1 inch wheel cylindar instead of the 1 3/16 I get a 41% increase directly to the rears.
Thanks,
TJ

No!!! If you use the 1" cylinders instead of the 1 3/16", you loose braking power...

Don't worry too much about pedal travel, make sure your brakes are well balanced...


I have a feeling that the rear 1 3/16 are used in combination with the larger calipers..

1 3/16 vs 1" = 41%
3" vs 2.5" = 44%

More power to the wheels and relatively proportional brake balance kept...

Quote:

I have a feeling that the rear 1 3/16 are used in combination with the larger calipers..

1 3/16 vs 1" = 41%
3" vs 2.5" = 44%

More power to the wheels and relatively proportional brake balance kept...

That's my thinking too. If I changed wheel cylinders or calipers, I would think doing both would be optimal so as to keep the relationship front to rear roughly the same.

So far I'm quite happy with the GMT800 MC, but I'm intersted to hear what some of you have to say about it if you decide to try it out. I'm wondering how the larger 3" calipers and larger 1&3/16" wheel cylidners would work in comination with the GMT 800 MC. This should exacerbate the smaller MC piston effect more, going to larger slave pistons on the other end, but even more of a good thing is not always better. There is an ideal balance here.... somewhere. Wasn't it GM's job to figure that out when they designed these trucks?

JR96CK wrote:

There is an ideal balance here.... somewhere. Wasn't it GM's job to figure that out when they designed these trucks?

Yep and ha!!

The goal here is to stop in a shorter distance and have a good pedal feel. I think you can verify your brake distribution by comparing your braking distance with ABS ON vs OFF.
While braking with ABS off, make sure you don't lock up your rears or you'll spin and possibly crash!
Gradually increasing the speed of the tests, brake at the limit where you start to hear your tire squeal without locking them...

Nice info:
http://stoptech.com/technical-support/technical-white-papers/white-paper---brake-bias-and-performance-why-brake-balance-matters

http://takemebeyondthehorizon.wordpress.com/2009/03/09/how-to-slow-down-faster-the-math-behind-brakes-part-5/

http://www.pirate4x4.com/tech/billavista/Brakes/#MasterCylinders


Gm seem to build cars by picking parts out of a parts bin.

And then fine tuning them with easy cheap fixes... aka prop valve.

But at 85/15 to me is conservative for an SUV and is probably more suited for an FWD or a P/U with a high front weight bias.

But then again it's not that simple for us to figure this ratio out just by calculating the weight transfer. I mean how do you compare the brake torque of a 12" disc vs a 10" drum...

Going from 1" to 1-3/16 slave cylinders increases the hydraulic leverage by 41% but how much braking force did you really gain?

Going from 2.5" calipers to 3" ones might makes it easier on your foot but if you use the same size pads and discs, your maximum braking power is still the same.

There's a limit to the braking power that a specific size brake pad material can generate...

So what' left if trial and error....

Hi...new guy here. Terry emailed me and asked me to chime in. I've been with Centric parts for about 10 years, and was a tech at GM and Toyota stores for about 5 years before that. I run the tech support/QC dept. for Centric, and I drive a '97 C1500. This is my 3rd GMT400.

I read through some of the posts here...and though I'd share some of my knowledge on this crap brake system.

GM evidently had a brilliant idea to reduce drag in an effort to increase MPG...one part of this idea was a "low drag caliper". The seal groove is machined a bit differently than a normal caliper such that the seal will retract the piston more than a normal caliper.....only problem is that this requires considerably greater fluid volume to get the piston back out of the caliper far enough to clamp the pads against the rotor. They solved this problem by developing a "quick take-up" master cylinder.

There is a picture posted on page 3 here of an original master torn down...the larger bore in the rear is the "quick take up" (qtu) feature. When the brake pedal is depressed, the large bore moves a high volume of fluid (bypasses over the seal in the small bore directly in front of it) with little pedal travel to get the piston out of the caliper relatively quickly....once this happens, and the system begins building pressure, there is a valve between the cylinder and the reservior that "pops off" (I don't know the actual PSI rating on this valve...but we can call it 20psi for this discussion) and bleeds all the fluid from the qtu chamber back into the reservior. The theory behind this is that once the caliper piston movement is done with, it will switch over to the smaller bore chamber in the master cylinder so that enough pressure can be created to sufficiently clamp the caliper down on the pad/rotor. Once the pressure on in the qtu chamber drops below the pressure of the smaller chamber, fluid will stop bypassing the seal and the brakes will operate off of the smaller piston....

This was a good theory, but poor execution. The pop off valve in the master I feel cannot pass enough fluid volume quickly, thus the pressure rise in the circuit is not proportional in any manner to the amount of pressure you put to the pedal. If you slam on the pedal in a panic stop situation....not a lot happens. If you depress the pedal just a bit and hold it for a half second or so, then apply pressure to the pedal...the brakes perform (and I use that term loosely) much better because you've given the qtu valve a chance to catch up and zero the pressure in the qtu chamber of the cylinder so that the master can effectively operate on the smaller bore.

Another problem, aside from the tiny rotors, is that the calipers are flexy weak pieces of crap. If you have someone apply the brakes hard with the engine running, you can see the caliper flex open like a clam shell. This poses 2 problems. The first one is excessive pedal travel/soft pedal feel. The second is that as soon as the caliper starts flexing, the pads are no longer parallel. When the caliper flexes..all of the clamping force of the pads ends up being concentrated at the very OD of the brake rotor...which tends to make brakes rather ineffective.

The solution, as I see it: You must use a pad that has fairly high initial bite...that can generate an appropriate amount of friction at relatively low clamping forces/line pressures. More importantly though, one simply must understand how this system works and adjust their driving style to accomodate it. Make a point not to just jump on the pedal...if you do, the QTU circuit will keep bypassing the small bore seal causing line pressure rise to occur very slowly...at which point the driver will continue to lean on the pedal, and by the time the QTU valve finishes doing it's thing and the line pressure is now up, the caliper has flexed open so far that the pads aren't really doing much against the rotor. Instead, give the pedal a little pressure, wait just a second or so, then apply a little more pressure. If the rest of the system is in decent working order, this results in brakes that work halfway decent.

Brilliant!! Glad to have your input.

That explains the 400 MC perfectly and what I've experience with the brakes from day one.

I've notice the feed hole in the larger 40mm chamber. Would increasing it's diameter enable more fluid to the pop valve and reduce that initial jam?

I'll take an other look at that valve but it really looked like a one way valve with no way for the fluid to go back up in the reservoir...

But this design still leaves us with very little fluid volume for the rear and as soon as the rears wear out a bit, they stop working...

http://my.cardone.com/English/Club/Products/Brakes/Protech/Training/brake1.pdf

http://my.cardone.com/English/Club/Products/Brakes/Protech/Training/brake2.pdf

http://my.cardone.com/English/Club/Products/Brakes/Protech/Training/brake3.pdf

http://my.cardone.com/English/Club/Products/Brakes/Protech/Training/brake4.pdf

http://my.cardone.com/English/Club/Products/Brakes/Protech/Training/brake5.pdf

http://my.cardone.com/English/Club/Products/Brakes/Protech/Training/brake6.pdf

http://my.cardone.com/English/Club/Products/Brakes/Protech/Training/brake7.pdf

http://books.google.ca/books?id=5rW-ppyB_ugC&pg=PA75&lpg=PA75&dq=quick+take-up%22+master+cylinder+problems&source=bl&ots=fcoybfjHnp&sig=21vTU3AD4frXQzwYmn3FbnKhlJM&hl=en&sa=X&ei=YEgcUIDCFsqm6wHC1IGoBQ&ved=0CFYQ6AEwAg#v=onepage&q=quick%20take-up%22%20master%20cylinder%20problems&f=true

According to this, a conventional MC shouldn't work with low drag calipers:

http://www.mpbrakes.com/uploads/documents/lowdrag.pdf

That article could have saved me a bit of typing....LOL.

The article is sort of right. A large enough master to overcome the piston travel wouldn't create enough pressure to stop the truck, and a master small enough to create the pressure wouldn't move enough volume to keep the pedal off the floor....or maybe GM just didn't try a 34mm unit.

Depending on the valve, it may be possible to modify. I've seen about 3 or 4 different versions of that QTU valve between different manufacturers..... The brake would perform better, but you'd still have a crap pedal.

The switch to a 34mm cylinder might be a double edged sword...but might not. It would be cool to get some pressure reading data logging going to put it all on paper....I'll look into that. On one hand, you're increasing the surface area of the pistons by about 26%...which will be your volume increase. You're also going to be creating about 26% LESS pressure than you would with the original master...which MAY BE OK, since the boosters are way to powerful anyway, and more pressure with these calipers doesn't really amount to much and simply isn't necessary. The proportioning will still be the same. The big question: Will the added volume be enough to run the low drag calipers? We can't calc the starting point(which is really unknown, seeing as how the QTU chamber output is dependent on how you apply the pedal, so we don't know our starting point). The 40mm qtu chamber can move 25% more volume than the 34mm master cylinder initially, but then the 34 mm master will move 26% more then the stock master once the QTU is out of the equation. They may balance out to work correctly. If the rears actuate too soon, one could always add a stiffer spring in front of the piston.

I would suspect that the pedal will have more free travel but be very firm. The benefit will come from predictability and control. With the 34mm unit (not because it's 34mm but because it's decent design) line pressure will be directly proportional to pressure applied to the pedal....which is much nicer to deal with. I have to read through the rest of this thread and see what everyone has done, along with their results. If you guys need to measure or test anything...we've got about 100,000 master cylinders in our warehouse...don't buy 'em, just ask and I can measure/compare etc.

Now we can overheat the little tiny rotors even faster

CrazyHoe wrote:

Ok black, I figured an easy way to calculate the increase or decrease in braking power from the MC without having to calculate the area of the pistons.
Same goes for the calipers and rear cylinders comparison.

And before you install your JB6 set-up, I would strongly suggest you try the 34mm MC. Easy, simple install.
Then, if you're still not happy with your brakes, you can go to the JB6



Lets say that you push on the brake pedal with a certain amount of force: Say 100lbs

Just remember that going to a smaller diameter MC piston will give you more braking power but also more pedal travel.
This will not affect your brake distribution much It will a bit because of the MC internal springs.

Same with going to a bigger diameter caliper piston/rear cylinder: it will give you more braking power but also more pedal travel.
This will directly affect your brake distribution!



How much is the difference going from 40 to 34?


Divide diameter B from A and square the result. That will give you the percent increase or decrease.

square (A/B)


So square (40/34)= 1.38 or a 38% power increase! But also 38% more pedal travel. (Or as if you're pushing with 138lbs on the 40mm MC)


Now going to the 3" calipers instead of the 2.5": 44% increase power to the front!!

You'll end up with a brake bias way to the front!!


Same with the 1 3/16" rear cylinders instead of the 1": 41% to the rear!! Regardless of the brake distribution valve!

1 1/16 would be nice... But there is a 1 1/8" model for a 27% increase.... Maybe too much...


I think using the GMT800 MC is a great upgrade because the extra volume going to the back takes care of the slack that the auto adjuster doesn't take.

You have to look at surface area, not diameter.

Indeed and had I known what a freakin' "quick take-up" master cylinder and "low drag caliper" was, you and I both would have done a lot less typing..

And knowing that someone at GM was on freakin' drugs when he design the pop off valve would have saved me a lot of headaches!!

That tiny pin size hole in the middle of valve is what is supposed to bleed the high volume in an emergency stomp.
Look at the size of that thing! It's so small, no wonder I didn't see it. But sure enough, on the other side, there's a spring and a ball.

But there's something else I noticed, next to the pin, there's a small bleed path. This means that if you were to slowly press on the brakes, the oil would bleed by here back into the reservoir and not bypass the piston seal. You could end up with the pedal slowly creeping to the floor. Which is something I notice while at stop lights.

Had they design this valve better, the pedal feel would have been so much better.

Pat@Centric wrote:

You have to look at surface area, not diameter.

I know, but I got fed up of calculating areas to figure out the surface ratios, so I took a short cut...

How much more area does a 40mm piston have vs 34mm?

Circle Area = pi x r²

40mm = 3.1416 x 400 = 1256.6
34mm = 3.1416 x 289 = 907.92

1256.6/907.92 = 1.384

The 40mm has 38.4% more area than the 34mm

Or..

(d1/d2)²

40/34 = 1.1764,
1.1764 x 1.1764 = 1.384 !! = 38.4%

Same result, much quicker to calculate.

Bleeding procedure with "quick take-up" master cylinder or step bore:

http://my.cardone.com/English/Club/Products/Brakes/Protech/Articles/Bleeder.asp

Welcome to the forum, Pat. Thanks for all the information; it certainly shines light on a lot of things about the original design.

Welcome Pat, and I'd like to ask you to chime in on the GMT800 brake thread I'm about to start.

So is the JD brake system master not a QTU design then? Why does the Hydroboost systems not have the low/spongy pedal fell and have less pedal travel? My impression with the low drag calipers is no matter what master you have you are going to have more pedal travel unless of course the QTU actually worked correctly.

Has anyone drove pre 95 GMT400 trucks? Seems to me I have never noticed this problem on them and I know they use a different booster not sure about master tho but Im pretty sure its different to