Master cylinder and ABS Delete

[thegawd]

I'm sorry my burb is actually JB6 I was going off of memory and I was wrong.

I dont know the awnser though....

 

[PlayingWithTBI]

I think mine says 6600 or 6400.

Look on your door jamb (if it's still there). My C1500 is 5600#

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[thegawd]

ya my Burb is 7300. my sierra is either completely faded or painted over. weird how my burbs is perfectly legible.

well it appears to be both. lol. this entire truck was painted and it wasnt done very well. they also did not color match the original color so whenever I sand it or paint it the correct color, summit/Olympic white it dosent match. at some point I will address that but for now I have like 6 cans of the "correct" color. dammit.

I took some acetone and I dont know if it could possibly strip this paint or not. I dont think so but I dont have any laquer thinner on hand as I usually do. but I also took some 500 grit sand paper in a drill attachment for buffing headlights and I was clearly removing paint. I burned through one spot and decided to stop and wait until I have laquer thinner. I'm pretty sure I can remove the rest with it.

ah well.

 

[Schurkey]

The easy way to identify JB3 brakes is the front rotor is solid not vented. It was also identified as JN3 in the service manuals.

JB/JN3 has vented front rotors, but they're thinner than JB5/6 front rotors. Same diameter, different thickness. The JB3 calipers have smaller pistons, the JB3 master has a 1" regular bore and a 34mm (?) step-bore. The JB3 booster is small-diameter, single-diaphragm "pancake" booster.

JB5/6 has thicker rotors, bigger pistons in the calipers, 1 1/8 bore master with a 40mm step-bore, and a huger vacuum booster. The larger-diameter pistons in the master cylinder make the pedal harder to push, so more power assist is required.

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notice theres conventional calipers and low drag calipers... that's another reason that these rpo codes are important... see that thread to learn the differences....

^^^ This is a BIG DEAL! JB/JD 3, 5, 6. VERY IMPORTANT when selecting a master cylinder. Low-drag calipers are engineered to retract the piston farther than regular calipers. The pads don't rub on the rotors like regular calipers, so the wheels are easier to turn when the brakes aren't applied = better fuel economy, reduced emissions.

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Low-drag calipers need a big "gush" of low-pressure fluid to move the pistons back up to the rotor surface, then the usual amount of high-pressure fluid to actually apply the brakes and stop the vehicle. THEREFORE, GM installs a special "step-bore" or "Quick Take-Up" master cylinder with a huge bore at the back to supply a gush of fluid, then the normal-size bore towards the front to supply high-pressure fluid.

Little Shop of Horrors sells a non-step-bore master cylinder, doesn't say a friggin' thing about low-drag calipers and how their master is totally not-compatible with them. They do talk about "upgraded" calipers, but not what's "upgraded" about them.

You can't expect the Little Shop of Horrors non-step-bore master cylinder to play nice with low-drag calipers. Same for the GMT800 master conversions.

Good stuff! I have vented front rotors, so it looks like I have JB5 brakes, and all I need for JB6 is to replace the rear drum assemblies!

As said, JB3 has vented rotors also. JB3 was only used in the first few years of GMT400 production. GM wised-up after '91--'92, something like that. Beyond that, yes, converting from JB5 to JB6 is just a matter of throwing away the leading/trailing rear brakes in favor of the 11.x Duo-Servo rear drums--the front brakes are the same between the two. There may be a difference in the combination valve also--but I don't notice any problems with my JB5 system using the JB6 rear brakes.

I started with JB3, then threw all the JB3 crap in the scrap bin. Got JB5 front brakes, master, and booster from one Treasure Yard truck, and a 9.5" axle with JB6 rear brakes from another vehicle. Essentially a conversion from JB/JN3 to JB6.

SO: for the original poster, you've got a non-step-bore master with too large of a regular bore; making for poor hydraulic advantage but great pedal travel. Your brakes suck because of the incorrect master cylinder. The incorrect, oversized-bore master cylinder also means your power booster isn't powerful enough, making the situation even worse.

But don't forget, there's plenty of OTHER potential problems with your brakes--binding caliper mounts, glazed pads, incorrectly-adjusted rear brakes (SUPER common with the leading-trailing shoe nightmare) partially-seized caliper pistons...and the list goes on.

Here's the sort of thing I find when I pull calipers apart. These are the JB5 calipers I got from the Treasure Yard, which I cleaned-up and used on my '88 K1500. The pistons weren't "stuck" but they didn't move freely like they should. When I was done cleaning them up, they work fine.

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The "crust" of scuzz on the piston seal. Crust peeled off with a fingernail leaving a usable rubber seal:

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[thegawd]

and that's the rest of the story! Cheers Schurkey.

 

[Smitty Jr]

My truck has JB5 and my brakes suck. I plan on doing the ABS delete with an adj prop valve and Cunningham Machine RR disc setup. More so for ease of maintnence. I do believe my booster/ master is failing and the front hoses need replaced anyway so I will replace the front and rear hoses at the same time.

I'm considering doing the same setup - Cunningham conversion, ABS delete. I'm curious how yours has worked out, if you've done it already. I'm also considering NBS/hydroboost swap, if needed after.

 

[Schurkey]

I don't understand why folks pay money for rear discs, when Treasure Yard rear drums do a fantastic job, and come attached to a better rear axle than most 1500s got.

There's also zero problem trying to keep a functional park brake.

God bless the 14-bolt semi-float with the 11.x Duo-Servo brakes.

The so-called "NBS" master cylinder is NOT SUITABLE for the low-drag front calipers of 3, 5, and 6 braking systems.

 

[MrPink]

I'm considering doing the same setup - Cunningham conversion, ABS delete. I'm curious how yours has worked out, if you've done it already. I'm also considering NBS/hydroboost swap, if needed after.

I have not done it yet and may not. My wheel setup may not clear the caliper.

 

[Smitty Jr]

I'll let you know how it works out for me. After reading all of this, I've decided to hold off on the MC swap until I have some experience with the new setup. Thanks man.

 

[DamHoodlum]

I have not done it yet and may not. My wheel setup may not clear the caliper.

My Little Shop Kit Cleared My 15" Rally Wheels With No Problem.

Stops Amazing when used with all the correct components