Master Cylinder for GMT400 - 11.6" rotors, 3.15" piston calipers, 11.15" drums - Z56/9C1/Z60 vs JB6/JD6

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Horntoad

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Firstly, I know brakes have been discussed a lot. And I’ve read a bunch and forgotten half of that. Want to see if I’ve got some of this straight and ask a question or two. Thanks for any more patience and time you have for this topic.

I’ve got a 1995 5.7L 2DR 4WD Tahoe LS. It came with JB5 brakes.

I’ve added the 3.15” front calipers and have (but have not yet installed) 11.15-inch x 2.75-inch duo-servo rear drums with the GM 1721425 wheel cylinder. Those 3.15”calipers are traditional (non-low drag), correct?

1. Did the Z56/9C1/Z60 Tahoes with the 11.15” drums use a 1” or 1 3/16” wheel cylinder; I’ve seen conflicting info (unusual to come across conflicting into on the web, I know)? Which part number? What about the JB6/JD6; what rear wheel cylinder did they use with the 11.15” drums? Is it accurate that both the Z56/9C1/Z60 and JB6/JD6 Tahoes had the 1” rear drum wheel cylinder?

*It can’t be as simple as there being of GM book that I’m missing that has model year RPOs and their corresponding parts?*

**My master cylinder is leaking.** I know I could rebuild it, but thinking I'd rather swap it out.

From what I’ve read it seems, to best match the brakes, I should go with the 1.25” bore master cylinder out of the police/limited. I’m looking to go with combos GM used, if possible.

2. Is it accurate that there is no difference (other than the reservoir and need for an adapter) between the GMT400 and GMT800 1.25” bore master cylinders?

3. But the 1.25” bore “police” (9C1/Z56) master cylinder came with QTU (quick take up) that is not functional; Is that a correct statement? Can the same be said of the “identical” GMT800 1.25” master cylinder?

But I’ve seen where some say the GMT800 was designed for disc/disc whereas the GMT400 for disc/drum. Would it also be accurate to say that some GMT800s with 1.25” master had disc/drum?

4. If the GMT400 and GMT800 1.25” master cylinder are in fact identical, other than larger, different style reservoir, and if it is true that both were designed for/installed in disc/drum setups by GM, any reason I would not use the GMT800 version?

5. Which master cylinder is the best match for a 1995 5.7L 2DR 4WD (disc/drum) Tahoe LS with 3.15” piston front calipers and 11.15-inch x 2.75-inch duo-servo rear drums with the GM 1721425 wheel cylinder? Is it the 1.25" bore master cylinder, and does it make any difference if I get the GMT400 or GMT800 1.25" master cylinder? Do I have the right/best match wheel cylinder and is it, in fact, a 1” bore?
 

Schurkey

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I’ve got a 1995 5.7L 2DR 4WD Tahoe LS. It came with JB5 brakes.

I’ve added the 3.15” front calipers and have (but have not yet installed) 11.15-inch x 2.75-inch duo-servo rear drums with the GM 1721425 wheel cylinder. Those 3.15”calipers are traditional (non-low drag), correct?
As soon as you start swapping parts, YOU become the "Brake Engineer".

3.15" = 80mm. Far as I know, this is not a low-drag caliper. Used with JD7 brakes 8,600 GVW on my '97 K2500.

There is an 86mm caliper, too. Used on ~11,000 GVW vehicles.


1. Did the Z56/9C1/Z60 Tahoes with the 11.15” drums use a 1” or 1 3/16” wheel cylinder; I’ve seen conflicting info (unusual to come across conflicting into on the web, I know)? Which part number? What about the JB6/JD6; what rear wheel cylinder did they use with the 11.15” drums? Is it accurate that both the Z56/9C1/Z60 and JB6/JD6 Tahoes had the 1” rear drum wheel cylinder? *It can’t be as simple as there being of GM book that I’m missing that has model year RPOs and their corresponding parts?*
GM parts catalogs would tell that tale. You could contact a Stealership and ask 'em.

**My master cylinder is leaking.** I know I could rebuild it, but thinking I'd rather swap it out.
Wise.

From what I’ve read it seems, to best match the brakes, I should go with the 1.25” bore master cylinder out of the police/limited. I’m looking to go with combos GM used, if possible.

2. Is it accurate that there is no difference (other than the reservoir and need for an adapter) between the GMT400 and GMT800 1.25” bore master cylinders?
The JN/JB3 used a "1-inch" master cylinder, 32 or 36mm (I forget which) Quick Take-Up piston. The JN/JB5 and 6 used a 1 1/8-inch master, also QTU but 40mm piston. Those are the only QTU master cylinders for GMT400 that I'm aware of. The 1.25 is probably not a QTU; so functionally the GMT400 and GMT800 should work the same...IF (big IF) they'll mount to the booster correctly, and connect to the brake tubes, and don't interfere with anything else.

"I" would not bother with a GMT800 master cylinder. There's no benefit to the newer design when used on the older bodystyle.

3. But the 1.25” bore “police” (9C1/Z56) master cylinder came with QTU (quick take up) that is not functional; Is that a correct statement? Can the same be said of the “identical” GMT800 1.25” master cylinder?
I'm not aware of a 1.25 master that has QTU. Far as I know, NONE of the GMT800s have QTU.

Again, the Stealership parts catalog would tell the tale.

But I’ve seen where some say the GMT800 was designed for disc/disc whereas the GMT400 for disc/drum. Would it also be accurate to say that some GMT800s with 1.25” master had disc/drum?
No idea.

4. If the GMT400 and GMT800 1.25” master cylinder are in fact identical, other than larger, different style reservoir, and if it is true that both were designed for/installed in disc/drum setups by GM, any reason I would not use the GMT800 version?
No reason to use the '800 cylinder in a '400 body style. Just asking for problems as far as I'm concerned.

5. Which master cylinder is the best match for a 1995 5.7L 2DR 4WD (disc/drum) Tahoe LS with 3.15” piston front calipers and 11.15-inch x 2.75-inch duo-servo rear drums with the GM 1721425 wheel cylinder?
You'll find out as you experiment. If those parts were ever used in an OEM application, I don't know about it.

But a 1.25 master cylinder bore seems reasonable. That's what's on my 97 K2500 with 80mm calipers. So it'd match the front, anyway. No promises on the pressure ratio of the 1.25 master to whatever bore diameter the rear cylinders are.

does it make any difference if I get the GMT400 or GMT800 1.25" master cylinder?
I don't understand the fascination with the GMT800 master cylinders.

Do I have the right/best match wheel cylinder and is it, in fact, a 1” bore?
You'll find out. Measure it to find the bore. Use it to see how it works. Change it for a larger- or smaller-bore cylinder to tune the brakes so they work best for the conditions you drive in--heavily or lightly loaded, for example.
 
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Supercharged111

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I've done the 800 master and it was a meager improvement, but it's really going about trying to improve the brakes the wrong way. It reduces pedal travel, and my theory for why most vacuum brakes suck is that they over-center. If you're going to get away from low drag calipers, do yourself a favor and get the correct master cylinder from our body style trucks. No matter what you do, hydroboost will make it better. A lot better. It uses a different pedal with different geometry. There's nothing wrong with being your own engineer as long as you know what you're doing and you know what you want.
 

1998_K1500_Sub

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Firstly, I know brakes have been discussed a lot. And I’ve read a bunch and forgotten half of that. Want to see if I’ve got some of this straight and ask a question or two. Thanks for any more patience and time you have for this topic.

I’ve got a 1995 5.7L 2DR 4WD Tahoe LS. It came with JB5 brakes.

I’ve added the 3.15” front calipers and have (but have not yet installed) 11.15-inch x 2.75-inch duo-servo rear drums with the GM 1721425 wheel cylinder. Those 3.15”calipers are traditional (non-low drag), correct?

1. Did the Z56/9C1/Z60 Tahoes with the 11.15” drums use a 1” or 1 3/16” wheel cylinder; I’ve seen conflicting info (unusual to come across conflicting into on the web, I know)? Which part number? What about the JB6/JD6; what rear wheel cylinder did they use with the 11.15” drums? Is it accurate that both the Z56/9C1/Z60 and JB6/JD6 Tahoes had the 1” rear drum wheel cylinder?

*It can’t be as simple as there being of GM book that I’m missing that has model year RPOs and their corresponding parts?*

**My master cylinder is leaking.** I know I could rebuild it, but thinking I'd rather swap it out.

From what I’ve read it seems, to best match the brakes, I should go with the 1.25” bore master cylinder out of the police/limited. I’m looking to go with combos GM used, if possible.

2. Is it accurate that there is no difference (other than the reservoir and need for an adapter) between the GMT400 and GMT800 1.25” bore master cylinders?

3. But the 1.25” bore “police” (9C1/Z56) master cylinder came with QTU (quick take up) that is not functional; Is that a correct statement? Can the same be said of the “identical” GMT800 1.25” master cylinder?

But I’ve seen where some say the GMT800 was designed for disc/disc whereas the GMT400 for disc/drum. Would it also be accurate to say that some GMT800s with 1.25” master had disc/drum?

4. If the GMT400 and GMT800 1.25” master cylinder are in fact identical, other than larger, different style reservoir, and if it is true that both were designed for/installed in disc/drum setups by GM, any reason I would not use the GMT800 version?

5. Which master cylinder is the best match for a 1995 5.7L 2DR 4WD (disc/drum) Tahoe LS with 3.15” piston front calipers and 11.15-inch x 2.75-inch duo-servo rear drums with the GM 1721425 wheel cylinder? Is it the 1.25" bore master cylinder, and does it make any difference if I get the GMT400 or GMT800 1.25" master cylinder? Do I have the right/best match wheel cylinder and is it, in fact, a 1” bore?

Lot's of questions here. Lots of good answers from lots of knowledgeable people, already.

My $0.02 follows, mostly in regard to your last (#5) question:

MCs: There appears to be two bore sizes available for the GMT400 in the aftermarket, 1.125" which seems to be applicable to vacuum systems and 1.25" which seems to applicable to hydroboost systems. Any MC with a "stepped bore" is a "QTU MC" and should only be used with a "LD caliper".

My suggestion: Use non-QTU MC b/c of your 3.15" calipers (which are almost certainly non-LD), and either 1.125" bore for vacuum (if vacuum is what you have) or 1.25" bore for hydroboost. The non-QTU MCs are likely to be listed as being for "over 8600 GVW" or something similar, as often found on heavier trucks (2500 and maybe 3500). You could use the GMT800 MC if you like; I used NBS MC ACDelco 18M1159on my K1500 Suburban and I'm happy with the results, but in hindsight I would probably use a GMT400 1.125" MC as the my NBS reportedly has a larger bore.... i.e., the ACDelco 18M1159 reportedly has a 1.339" (34mm) bore.

Rear cylinders: The p/n you mentioned appears to be a 1" cylinder. These were OE on my 1998 K1500 Suburban w/ JB6 brakes and may function well for. I replaced mine with the 1.1875" diameter cylinders (ACDELCO 18E1362, OE for 13" DRW). My and all Suburbans are a$$-heavy (relative to their unloaded, pickup-truck counterparts) and this change really helped the braking IMHO.

FWIW, my Suburban is ~3020# on the front, ~#3160 on the rear, ~6180# total.
 
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I have a 1999 K1500 with what I assume are JB6 brakes. I upgraded to a GMT800 Master Cylinder a couple of years ago and didn't see much difference.

However, I recently installed a new GMT400 vacuum servo and now have way too much pedal travel before the master picks up and I get brake engagement.

Is there a push rod adjustment on the vacuum servo or is this a consequence of a mismatched servo booster and (GMT800) master cylinder ??? Or the GMT800 MC not having rapid take up ?

I've seen HydroBoost Upgrade mentioned several times. Does anybody have a parts listing for this ? (For the JB6 Brakes)
 

Schurkey

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It's possible you have JB6, but not likely. I'd expect JB5, which is the same as JB6 at the front and middle, but has a lighter-duty rear brake system. JB5 has the terrible 254mm rear drums, JB6 has the 11.x rear drums.

Check your SPID in the glovebox to be sure.
 

HotWheelsBurban

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Yes in my experience the only trucks that have JB 6 for sure are Burbs.... I've seen the bigger brakes(11") on some Tahoes that were 4x4, but most likely that half ton pickup is going to be JB 5. As noted, check the service parts ID sticker (in the glovebox) or have your VIN number run(by @1ton-o-fun )
 

L31MaxExpress

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My 97 Express has a mix of brake parts that are working well currently. I do plan to rebuild the rear brakes soon.

Currently have a matched hydroboost and master cylinder for a 2007 3500 van. Master cylinder has a 1.43" bore. Front calipers are matched to the front rotors. They are 2001-2002 Express application. Rotors are 12.47" diameter, 39.1mm thick and the caliper pistons are 3.00" up front. The rears are currently 13 x 3.5" drums with a 1" wheel cylinder. Since I am 3,600# over the rear axle empty, I am going to up the rear wheel cylinder to a 1.1875" bore when I have it all apart.
 
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