JB5 JB6 JB7 JB8 Size Thread

[Schurkey]

Hello,

After hours of looking around i can't find a thread that list all the sizes of the various brake components of JB style brakes. Please let me know if what i have so far is correct. I'm sure i have things wrong as all info is coming from other threads.

JB5(6400lbs):
1.574" Bore MC
Low Drag Calipers 2.935" Piston
Front Disc- 11.86x1.29
Rear Drum- 10x2.25
Vacuum- Double Diaphram
1 3/16" wheel cylinder

JB6(7200lbs):
1.574" Bore MC
Low Drag Calipers 2.935" Piston
Front Disc- 12.50x1.28
Rear Drum- 11.15x 2.75
Vacuum- Double Diaphram(same as JB7)
1" rear wheel cylinder. 11' drum has self activating shoes, therefore does not require as much pressure as the 10"

JB7(8400lbs)Single rear wheel:
1.25" Bore MC(used in JB5 and JB6 Hydro Units)
Conventional Calipers 3.15" Piston
Front Disc- 12.50x1.54
Rear Drum- 13.00x2.50
Vacuum- Double Diaphram(same as JB6)
1 1/16 rear wheel cylinder

JB8(10000lbs)
1.312" Bore MC
Conventional Calipers 3.4" piston
Front Disc- 12.50x1.54
Read Drum- 13.00x3.50
Hydraulic Hydro Boost
1 3/16 rear wheel cylinder

First Guess: This varies by year or perhaps by series.

My '97 K2500 has JD7. JD7 in '97 uses hydroboost. According to my '97 service manual (Bishko, on DVD)
JB5 = vacuum
JD5 = Hydraulic
JB6 = Vacuum
JD6 = Hydraulic
JB7 = Vacuum
JD7 = Hydraulic
JB8 (single wheel) = Hydraulic
JB8 (dual wheel) = Hydraulic
JF9 = Hydraulic

All of the xx5 and xx6 use 11.57 x 1.25 rotors in front.
xx5 uses leading/trailing drums
xx6 uses 11.15 x 2.75 Duo-Servo drums

xx7 and xx8 use 12.5 x 1.26 rotors except JB8 (Dual wheel) that has 12.5 x 1.5 rotors
xx7 has 13 x 2.5 rear drums
xx8 has 13 x 3.5 rear drums

JF9 has 13.86 x 1.435 front rotors and 13.58 x 1.435 rear rotors

My chart doesn't show master or wheel cylinder bore sizes, or whether they're standard- or quick-take-up calipers.

 

[HotWheelsBurban]

Something else to add to this:C1500 Suburbans got JB6 brakes, 11 5/32" rear drums. Pickups are more likely to have JB5. Not sure what Tahoe/Yukon/ Blazer/ Jimmy would have, since I've not taken one apart. Something else to look at next wrecking yard trip.....

 

[454cid]

...xx8 use 12.5 x 1.26 rotors except JB8 (Dual wheel) that has 12.5 x 1.5 rotors

Some SRW trucks (extended cabs, according to what I've seen on Rockauto) and later SRW vans (seen in person) also got the thicker rotors and bigger calipers. This is what makes it easy to swap the DRW front brakes onto SRW trucks. 4x4's just use the DRW truck rotors and 2wds should be able to use the van rotors.

The DRW JB8 drums got larger wheel cylinders..... I want to say 1-3/16" instead of 1-1/16". I've got the front swapped to the DRW parts, but haven't done the drums yet.

 

[Erik the Awful]

So, when I get around to ditching my JB5 brakes, what's the best vacuum brake setup to look for in the junkyard? JB7?

** Note that I am not interested in going to hydroboost, nor in an NBS booster swap. **

Do I need new front spindles?
Can I swap the JB7 drums onto the 10-bolt?
Did they make a 5-lug 14-bolt with JB7 brakes?

 

[arrg]

So, when I get around to ditching my JB5 brakes, what's the best vacuum brake setup to look for in the junkyard? JB7?

** Note that I am not interested in going to hydroboost, nor in an NBS booster swap. **

Do I need new front spindles?
Can I swap the JB7 drums onto the 10-bolt?
Did they make a 5-lug 14-bolt with JB7 brakes?

JB6 is the best you can do with 5 lug. The JB7 stuff is 8 lug. If you're JB5 now, All you need is a different rearend with the JB6 drums.

 

[Steven Petersen]

JB6 is the best you can do with 5 lug. The JB7 stuff is 8 lug. If you're JB5 now, All you need is a different rearend with the JB6 drums.

I swapped a 9.5” 6 lug axle into another 88 K1500 I owned 25 years ago. It hooked right up to the existing brake system and the braking improvement was huge. I never realized it was actually mismatched to the 1/2 ton system until now. But I can verify from personal experience it works excellent.

 

[HotWheelsBurban]

So, when I get around to ditching my JB5 brakes, what's the best vacuum brake setup to look for in the junkyard? JB7?

** Note that I am not interested in going to hydroboost, nor in an NBS booster swap. **

Do I need new front spindles?
Can I swap the JB7 drums onto the 10-bolt?
Did they make a 5-lug 14-bolt with JB7 brakes?

Find a Burb in the yard, the JB6 stops my Burb well even with a load. So should be great on a pickup that's 1000 pounds lighter than the Burb ( built in camper top and extra seating).
The square body GMC pickups we had for our parts store delivery trucks were Heavy Half, and they had the bigger drums out back like JB6 on the 400s. Probably better front brakes too. But I remember those big drums, with the #473 shoes. Stopped those pickups on a dime and gave you change! They were a 78 and an 81,both V8s and long bed. Dad built a heavy wood tray in the bed of each truck, with 2x8 sides and 3/4 plywood base, and wood bins in front of and behind the wheel wells. The main tray had a couple of removable dividers, so you could put stuff in the bed and it didn't slide all over. So they had a few hundred pounds in the bed for traction.....

 

[Jerryred94silvy]

JB6 is the best you can do with 5 lug. The JB7 stuff is 8 lug. If you're JB5 now, All you need is a different rearend with the JB6 drums.

Find a Burb in the yard, the JB6 stops my Burb well even with a load. So should be great on a pickup that's 1000 pounds lighter than the Burb ( built in camper top and extra seating).
The square body GMC pickups we had for our parts store delivery trucks were Heavy Half, and they had the bigger drums out back like JB6 on the 400s. Probably better front brakes too. But I remember those big drums, with the #473 shoes. Stopped those pickups on a dime and gave you change! They were a 78 and an 81,both V8s and long bed. Dad built a heavy wood tray in the bed of each truck, with 2x8 sides and 3/4 plywood base, and wood bins in front of and behind the wheel wells. The main tray had a couple of removable dividers, so you could put stuff in the bed and it didn't slide all over. So they had a few hundred pounds in the bed for traction.....

So the front portion of the brakes would swap right over, but would you actually have to swap the whole rear end for just the drums? I’ve been looking at ways to improve the brakes on my ‘94 2wd and I’m leaning towards the hydro boost swap and, after reading all this juicy knowledge, swapping to JB6 equipment. Or would it be JD6 with the hydraulic system?

 

[Schurkey]

So the front portion of the brakes would swap right over,

Swap? I bet you've already got the JB5--JB6 front calipers, rotors and master cylinder. You could add the hydroboost system.

The question is whether you end up with 5- or 6-lug wheels and hubs.

but would you actually have to swap the whole rear end for just the drums?

You need everything from the backing plates outward, not just the drums. I think the JB6/JD6 backing plates would bolt up to the 8.5" rear axle. Given that the 8.5" rear axle is light-duty, you might as well just slap a 9.5" axle under the back--all it takes is a conversion joint and U-bolts on the K-series; I guess you'd need the conversion joint, U-bolts, and the 6-lug wheels on a C-series.

I’ve been looking at ways to improve the brakes on my ‘94 2wd and I’m leaning towards the hydro boost swap and, after reading all this juicy knowledge, swapping to JB6 equipment. Or would it be JD6 with the hydraulic system?

Far as I know you've already got the best part of the JB5 system--the front calipers and rotors along with the larger-bore master cylinder and bigger (vacuum) power booster.

It might be worth your time to assure that those calipers are in good condition, and floating freely on the caliper mounts. Same for pads and rotor. The calipers I used really needed to be taken apart and cleaned-up; which I did. I replaced the master cylinder with a Delco new unit (Communist Chinese--very disappointing even though it did fix the fluid leak at the reservoir-to-master cylinder seal) and flushed the fluid in front. I still need to dick with the rears; but they did get flushed when the rear axle was changed to a 9.5" with the bigger duo-servo drums.

Yeah, getting rid of the leading/trailing rear brakes is another worthwhile upgrade (but use the whole axle if you find one in good condition/correct width/correct gear ratio!)

I still have some ambivalence about hydroboost. But then my ONLY experience with it is on a vehicle that I haven't done any brake service to; so I might be driving a crappy hydroboost system--not getting the full potential.

 

[Jerryred94silvy]

Swap? I bet you've already got the JB5--JB6 front calipers, rotors and master cylinder. You could add the hydroboost system.

The question is whether you end up with 5- or 6-lug wheels and hubs.


You need everything from the backing plates outward, not just the drums. I think the JB6/JD6 backing plates would bolt up to the 8.5" rear axle. Given that the 8.5" rear axle is light-duty, you might as well just slap a 9.5" axle under the back--all it takes is a conversion joint and U-bolts on the K-series; I guess you'd need the conversion joint, U-bolts, and the 6-lug wheels on a C-series.


Far as I know you've already got the best part of the JB5 system--the front calipers and rotors along with the larger-bore master cylinder and bigger (vacuum) power booster.

It might be worth your time to assure that those calipers are in good condition, and floating freely on the caliper mounts. Same for pads and rotor. The calipers I used really needed to be taken apart and cleaned-up; which I did. I replaced the master cylinder with a Delco new unit (Communist Chinese--very disappointing even though it did fix the fluid leak at the reservoir-to-master cylinder seal) and flushed the fluid in front. I still need to dick with the rears; but they did get flushed when the rear axle was changed to a 9.5" with the bigger duo-servo drums.

Yeah, getting rid of the leading/trailing rear brakes is another worthwhile upgrade (but use the whole axle if you find one in good condition/correct width/correct gear ratio!)

I still have some ambivalence about hydroboost. But then my ONLY experience with it is on a vehicle that I haven't done any brake service to; so I might be driving a crappy hydroboost system--not getting the full potential.

The front calipers are good, I went through them when I did pads and rotors not too long ago and the rear brakes looked alright when I was under it last. Could a fella swap the rear end and keep my 5 lug axle shafts? I’d like to not have to buy new 5 new rims at the moment lol.