Pat, thanks for chiming in. This really is a great bunch of talented guys here and have accomplished some pretty unique things with these trucks. Thanks for sheding some light on this brake design!
TJ

It looks like less pressure on these calipers is a good thing since they are so weak. Next question are the 3/4T -1T calipers just as weak?
TJ

Quote:

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

My '88 and '92 were the same as my '98 as far as pedel travel and feel. I notice as well that at a stop light my pedal will slowly sink. I didn't know why up until now! It looks like the caliper is interchangeable from '88-'99 so It confirms for me this has been a design since'88.

My '84, '86, '91, and '93 S10s were the same feel. My 2000 S10 Zr2 seemed to have better stopping results.

Come to think of it that s10 ZR2 was a hybrid of sorts, I wonder what calipers were used on that setup?

TJ

98Blackss wrote:

It looks like less pressure on these calipers is a good thing since they are so weak. Next question are the 3/4T -1T calipers just as weak?
TJ

I was wondering just that. I have a set. Thinking of putting a dial indicator on them to measure the actual flex.

BTW, R auto sell the prop valve for the jb5 and jb6 so the pressures must be different.


This low drag caliper thing sucks. I hate the QTU master. I want a firm predictable powerful pedal.

Looks like a combination of 1-3/16 rear and 3.384 front would be a nice combo for a bit more rear bias.

And the d369 pads would be for the 1500 calipers, the d370 pads for the 2500+ calipers.

R-A caliper piston listed for 3500
3.147"
3.384"
80MM

Tahoe
2.935"
3.147" (Police package)


3 pads listed:
d369
SIZE A: 126X76.5X16.8mm
SIZE B: 147.3X59.4X15.8mm



d370
SIZE A:125.7*85.6*17mm
SIZE B:147.3*58.9*15.7mm


d459
125.7*84.6*17.2mm
147.4*58.9*16.5mm

The express 2500's have the 1-1/16 rear cylinders.

Add one more to the discussion.

The GMT900 (disc/drum) MC bore is about 1.125". I will try to take it apart later and get better measurement with my calipers when I go to my garage.

How would this MC act with the Low drag calipers.

Pic:

99bluetahoe wrote:

Add one more to the discussion.

The GMT900 (disc/drum) MC bore is about 1.125". I will try to take it apart later and get better measurement with my calipers when I go to my garage.

How would this MC act with the Low drag calipers.

Pic:

The GMT900 MC does not bolt up to the GMT400 vacuum booster. The bolt holes are spaced out two far. So forget about what I posted.

Since I have hydroboost and swapped to a 1.25 bore master cylinder, I wonder how much of the better braking is result of the MC and not the booster. As mention by speeder, the Police/Limited(performance pkg) Tahoes had a different master cylinder. Doing some looking, ACdelco part # 18M712 comes up as a replacement part for this version of the tahoes. This MC cylinder also looks to be a 1.25" bore non-QTU MC same as the hydroboost and the SSBC Firm-feel MC. The police/limited tahoes had JB6 rears but the same low-drag front caliper system. If GM used these parts together from factory then they must have thought this regular MC was fine with the low-drag calipers.

Anyone know of a 1.250" bore MC from a gmt800 truck like the one ssbc is using.

Are we sure the GMT800 master doesent have the QTU valve in it? I would assume since these trucks are even newer than ours they would also have low drag calipers since fuel economy is getting to be a bigger factor these days. Now I know the 800s use 2 piston calipers and different type of slide pins than our trucks do that probably drag less than our hunks do but just a thought. Makes me want to pull some calipers apart and see what this supposed seal is that makes it spring back so to speak.

Bottom line the 800 master takes care of the shit feel pedal and fits with virtually no modifications so whats the big deal all about?

Whipped383 wrote:

Makes me want to pull some calipers apart and see what this supposed seal is that makes it spring back

No need, it's all here

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

Ok that expains that, I like where it says "If you don’t use a quick take up master with
low drag calipers you will never get a good pedal"

The S10s all had 63.5mm calipers until they went to the dual piston in about '98.

The seal itself is not the problem in the GMT400 caliper...it's the angle cut into the seal groove that lets the seal flex more.

The GMT800 does not use a QTU valve....nor do they use low drag calipers. GM realized the error in their ways and stopped when they went to he 2 piston calipers. A quick take up master cylinder will have a larger diameter chamber near the booster, which the later cylinders do not have..they are straight bore.

The Police Package tahoes used the same master cylinder (same size at least) and the same calipers as the 8600GVW trucks (3/4 ton).

There are 3 sets of single piston calipers, and 3 different pads that go with them.

D369 is used with the 75mm caliper (1/2 ton) .... this setup all used one master cylinder
D370 is used with the 80mm caliper (3/4 ton and police tahoe) .... There are 2 master cylinders that were used on these applications...I just looked at the least popular one and it appears to be a straight bore. I'm checking this out now.
D459 is used with the 86mm caliper (1 ton)

The 80mm caliper and the D370 pad WILL fit onto the 2WD C1500...and will work fairly OK with the stock master. I ran this setup on my old truck before I really understood the system like I do now. It was better than stock, but still sucked.
This setup WILL NOT fit on the 6 lug 4x4 trucks...the pad is a bit wider and it will hit the rotor hat. Especially when using a modern type full cast rotor with a thicker hat. They may work with the original "composite" rotor that used thinner material in the hat. I'll post up again after I check out that master cylinder

AC Delco consolidates the 2 different 1.25" master cylinders for the GMT400 police package tahoe as well as the C2500 (with 13" rear brakes and 80mm calipers). These cylinders, externally, appear to be dual bore QTU style cylinders, but they are not. They are 1.25" straight bore cylinders...but appear to have a QTU valve in the reservior. I'm not sure what purpose this serves. Probaby none...I imagine they just left it there so that they could consolidate castings/reservoirs. I've got an excellent pad/rotor combination I'm going to be testing on my truck here in the next week. After I baseline that setup, I'm going to try the C2500 master cylinder with the 75mm calipers, then I'll try the C2500 master cylinder with the 80mm calipers. I'll try to post results in the next couple weeks.

This thread is full of win! I can't wait to hear the results...after driving GMT900's for the last 5 years I'm back in a GMT400 and the first thing that makes me cringe is the brakes, right above the steering (another story). I've heard that the GMT800 master cylinder from the vacuum boost, disc rear brakes corrects the pedal feel issue but has too much rear bias, does that jive with your research? I'm thinking of trying a GMT800 unit from around 2005 when drums were re-introduced in combination with the 3/4 ton calipers...or I can just wait on your experiment! I like to drive my trucks like go carts, right now it's more like driving a Rascal power scooter at Wal-Mart.

Leevon

no master cylinder, of any size, should change the Bias. We're essencially going from a 29mm cylinder to a 34mm (when using the GMT800 cylinder) or 31.75mm when using the one I'm going use, from the 3/4 ton GMT 400. The only reason things may change is due to the unpredictable pressure build of the QTU master cylinder. There will be a "correct" size master cylinder. It's a balancing act....has to be big enough to run the caliper pistons out while being small enough to make pressure. Even with the QTU valves in the 31.75mm cylinder that I'm going to try, it won't react near as bad as the stock cylinder because it's all one size.

One thing to note...the 10" rear drum setups use a bigger wheel cylinder than all the others at 1 3/16". The 11" setup only uses a 1" wheel cylinder, and the 13" 8 lug setups use a 1 1/16" cylinder. Going to a larger, non dual bore/QTU master will result in less pressure at the rear, with faster pressure build for the front circuit......in theory.

I disagree with rear discs on these. Duo-Servo drum brakes are some of the most efficient brakes on the planet...they will outstop a disc setup any day of the week (wheel cylinder has a less than 1" of surface area per piston while calipers for a similar sized disc system will be 5-6 square inches to acheive the same result)....the problem is, they can't handle the heat. If rear drum heat isn't causing problems for you, the "upgrade" to discs I think is not of any advantage, but more than likely will result in a disadvantage. Go hop in a '85 chevy truck...same basic setup but without the stupid low drag/QTU parts...they stop great. If our trucks stopped as good as the trucks that were 10 years older, I don't think we'd have threads like this.

Duo-Servo drum brakes

Pat@Centric wrote:

One thing to note...the 10" rear drum setups use a bigger wheel cylinder than all the others at 1 3/16". The 11" setup only uses a 1" wheel cylinder, and the 13" 8 lug setups use a 1 1/16" cylinder. Going to a larger, non dual bore/QTU master will result in less pressure at the rear, with faster pressure build for the front circuit......in theory.

Crap! That could be the source of my front brake fade!

When I bought the truck, I redid all the brakes. The parts guy sold me a 1" wheel cylinder for my 2 Dr Tahoe... Should have been 1 3/16.

And I always found that the "teeth" on the self adjuster star wheel where big, needing lots of play before grabbing the next tooth.

Possibly more play than the 28.6 Master piston could compensate for.... Leading to a stop in self adjusting.

I had found that applying both the parking brake and brake would help it adjust bu not always...


And if the OEM low drag calipers need a QTU master, it would be cool if a company offered a modified non-flex seal (for a low drag caliper) for use with a strait bore Master.
Or would a pre-94 caliper fit?

But then if these calipers flex so much, maybe going to SSBC caliper is the way to go.... Now, are the SSBC calipers "low drag types as well"?


The SSBC combined pistons surface area equal a 76 mm single piston... But spread the load more evenly...



Last edited by CrazyHoe on Tue Aug 07, 2012 7:43 am; edited 1 time in total

What about the hydroboost master does it fit on the vaccum booster? And does it have the qta valve? What about piston size? No one with a hydroboost has the low pedal problem that Im aware of what the crap is different other than the booster?

I wasn't suggesting disc brakes are superior, it may read like that but I was trying to say somebody used a m/c from a truck with rear discs...and the rear drums locked up early, hence my mis-used term "bias". The disc/drums on my '71 chevy work as well as anything on the road, I don't get the fascination with rear disc conversions either and I believe it's an un-educated position to assume they outperform drums.