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Old 10-29-2005, 11:29 PM   #1
tch777
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What is the Difference, JB5, JB6 and JB7?

I found out I have a JB5 brake system in my and I have always thought its brakes were kinda weak, even after replacement and going to a shop for power bleed and adjustment.

I know the ratings for the different brakes but what hardware is different between these systems:
JB5 BRAKE, POWER, DISC/DRUM, 6400 LBS
JB6 BRAKE, POWER, DISC, DRUM 7200 LBS
JB7 BRAKE, POWER, DISC, DRUM 8400 L85
For example what can I install / replace in my JB5 system to make it a JB7?

Thanks

Tim
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Old 10-29-2005, 11:38 PM   #2
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According to the Chevrolet 10-30 series light truck service manual:

JB5= front disc 11.86X1.29 rear drum 11.15X2.75 Vacuum-Double Diaphragm

JB7= front disc 12.50X1.54 rear drum 13.00X2.50 Vacuum-Double Diaphragm

JB8= front disc 12.50X1.54 rear drum 13.00X3.50 Hydraulic-Hydro-Boost

JB7 is basically Dana 60 Front disc with the smaller 14FF drum brakes with a vacuum booster and JB8 is Dana 60 front disc with the Large 14FF drum brakes with Hydro-boost.

Ira
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Old 10-30-2005, 09:31 AM
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from what I can tell, you should be able to swap in a booster and master from a JB7 truck to replace the JB5 system. Just The masters alone are not interchangeable, but with the boosters should be. The master for the JB7 system is also the same master used in 1/2 and 3/4 ton hydro systems from what I can tell.
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Old 10-31-2005, 11:42 PM
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Can anyone verify applications or which vehicles are donor vehicles and what all I need to swap?

Thanks
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Old 11-01-2005, 08:59 AM
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When I converted to 1ton I went to autozone and they cross-referenced all of the different styles of JB applications. My donor vehicle was a diesel but the hydro boost was trashed so I went with the K20 booster and master and all is good.
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Old 11-01-2005, 11:36 AM
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Quote:
Originally Posted by BigCountryx
from what I can tell, you should be able to swap in a booster and master from a JB7 truck to replace the JB5 system. Just The masters alone are not interchangeable, but with the boosters should be. The master for the JB7 system is also the same master used in 1/2 and 3/4 ton hydro systems from what I can tell.
I swapped to a jb7 master cylinder with out changing booster mostly because i don't have the jb7 if i did i would change it right away for the proper one, now not trying to steel or hijack this post but right after i swaped to the jb7 booster on my burb it seems like i have little to no front brakes but i have so much rear brakes that i can skid my 33's on dry pavement at 60 with no problems, so any ideas if this is because of the brake bootser being from a jb5 or is it something else.

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Old 06-21-2013, 07:13 AM   #7
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Using an old thread.

What do Blazers come with? I'm curious if I have the JB5 or JB7 setup. But here's the catch, my rears are disc and I'm wondering if they are the larger calipers. I think they are too large for the brake lines, meaning I can't feed the larger pistons with enough pressure before my pedal bottoms out.

Here are some pictures. I've done all kinds of stuff to the system. But I'll start a new thread for any diagnosing. Just wondering what I have. Thanks

Stock booster with vette master.









Rear calipers. If it's even possible to tell from a picture.

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Old 06-21-2013, 07:27 AM
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the difference is the size of the cylinder in the master cylinder. get the biggest one you can find. you should be able to search for my old posts and find the part #.
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Old 06-21-2013, 09:31 AM
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If your pedal bottoms out, it is because of volume, not pressure. Having 4 big calipers in the system requires a lot of fluid flow to engage the brakes and the MC is barely able to provide it. Swapping to a bigger MC piston will give you more volume and therefore a higher pedal, but it also increases the pedal effort. Chances are it will be a good swap because MC piston size should logically increase with caliper piston size for the same sort of effort/travel. If you ended up with too much pedal effort, the only option then is to go hydro because that booster gives more boost. You could also pick some intermediate size MC.

You can also decrease pedal travel by using the brake pedal from a hydroboost truck (again, increasing pedal effort). But the MC is stroking just as much. When the MC bottoms out, that's it - no amount of effort will apply more brakes.

My experience in a nutshell, FWIW: swapping to rear discs gave a ton of pedal travel and a "whooshing" MC. Tried a replacement master and then a cadillac disc/disc master. All sucked. Never tried a 1-ton MC with the vacuum booster. Converted to hydroboost and then the pedal travel, effort and braking power were all great. That means a bigger MC, different booster and diffrent pedal geometry all at once, so I can't comment on how each piece would have done by itself.
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Old 06-21-2013, 09:46 AM
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Get a bigger master cylinder.

On my k5 I'm running the stock vac booster with a '99 C3500 master cylinder and 4 D60 calipers. I have great pedal feel and stopping power.
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