JB5 Rear to JB6 Drums 6 Lug - Help with Models to pick

[1998_K1500_Sub]

For the rear, I don't want to do an axle swap.

If you don't want to do an axle swap, you're stuck with the 10" drums. The 11" brakes cannot be easily swapped from a JB6 axle (e.g., from a junkyard) onto a JB5 axle, as the bolt patterns (and perhaps a few other things) at the end of the axle are different JB5 vs. JB6. The easier route is to swap an entire rear end.

There are disc brake "upgrades" for the JB5 axles. Some are junk, and some are better, e.g., see post https://www.gmt400.com/threads/little-shop-mfg-rear-disc-installed.48588/post-1252997 and others on the forum (@Schurkey often posts to the forum regarding brakes, he's a good guy to follow). If you get a disc brake conversion kit, get one that's got newer-style calipers mounted on integral brackets (as I noted in my post above) and not a cheapie kit that uses a the older-style (70s era) GM calipers. You'll often see me and @Schurkey in the forums speaking-out against those cheapie kits.

(edit) This kit, for example, used the newer style calipers and has a parking brake provision. I'm not endorsing it, I'm jus' sayin'...

https://www.cjponyparts.com/master-power-legend-series-disc-brake-conversion-kit-6-4wd-chevrolet-gmc-c-k-trucks/p/RDBC3/

 

[Caman96]

If you don't want to do an axle swap, you're stuck with the 10" drums. Well, you have the option of installing an aftermarket disc brake setup; some are junk and some are better. That topic's been covered elsewhere on GMT400.

Most who do the rear disc swap say other than ease of maintenance, there is no improvement in braking.

 

[1998_K1500_Sub]

I have already upgraded the MC.

I just noticed this ^^^

Depending on the MC you've used (quick-take-up or not) you'll be better served if you use the corresponding calipers, e.g.,

- quick-take-up MC is used with low-drag calipers typical of JB5/JB6, having 76mm pistons

- "regular" MC (e.g., the MC used on the JB7+ brakes for GMT400 K2500-3500s or the NBS MC, both of which are not quick-take-up designs) is used with "regular" calipers (JB7+) available w/ piston diameters of either 80mm or 86mm.

 

[Schurkey]

I have a 91 K1500 with the crappy JB3 brakes.

You have my sympathy. They're pathetic. That's what my '88 K1500 was born with.

I have already upgraded the MC.

As said...those are scary words depending on whether you actually downgraded with a "NBS" master.

OTOH, if you upgraded to the 1 1/8" Quick Take-Up master cylinder, you'll also need the larger JB5--JB6 vacuum booster, or still more upgrading to the Hydroboost.

For the front, l plan to keep it simple and swap over the hub/rotor/calipers to a JB5.

Good plan.

For the rear, I don't want to do an axle swap. My truck is just a DD that will never tow or be driven “hard”, so I am leaning towards just swapping to the better 11” drums. My question is - did the trucks that came with 11” drums use the same size brake lines?

As said...it's easier to just swap the entire axle, assuming you can find one with the right gear ratio and in good, usable condition. One U-joint, four U-bolts with hardware, and whatever you need to connect the brake plumbing, which might be the tubes already on the axle, or perhaps new tubes/new brake hose. I used the brake tubes that were on the axle I swapped-in for years. (They're replaced now, along with the rear hose.)

The diameter of the brake tubing would be the same. The bending 'n' forming is probably different due to the longer reach from axle-tube to the wheel cylinder based on the larger brake backing plate diameter. That is, you could maybe rework the brake tubes for the 254mm brakes to work with the 11.x brakes, but it might not be pretty.

There ARE 11" brakes intended for the 8.5" axle, but they're not real common, I guess. How well-stocked are the Treasure Yards around you?

 

[Caman96]

There ARE 11" brakes intended for the 8.5" axle, but they're not real common, I guess. How well-stocked are the Treasure Yards around you?

I don’t believe the 8.5 6lug 11” backing plate will bolt up to a 8.5 w/10” brakes. The backing plate for the 8.5/11” brakes is the exact same backing plate used on a 9.5.
I’m guessing the 8.5/11”brake rear ends have a different flange on them than typical 8.5’s with 10” brakes.

 

[1998_K1500_Sub]

There ARE 11" brakes intended for the 8.5" axle, but they're not real common, I guess. How well-stocked are the Treasure Yards around you?

I've got the 11" brakes on my L31 K1500 Suburban 8.5" axle. I suspect a lot of L31 K1500 Suburbans have them. But, as @Caman96 notes...

I don’t believe the 8.5 6lug 11” backing plate will bolt up to a 8.5 w/10” brakes. The backing plate for the 8.5/11” brakes is the exact same backing plate used on a 9.5.
I’m guessing the 8.5/11”brake rear ends have a different flange on them than typical 8.5’s with 10” brakes.

This jibes with my understanding, but I have no proof in the form of pictures or personal experience.

 

[Schurkey]

OK. I was wrong.

That is, there are 8.5" axles with 11.x brakes, but they don't swap to the axle he has.

So in the end...replacing the axle will be the best solution.

 

[1998_K1500_Sub]

That is, there are 8.5" axles with 11.x brakes, but they don't swap to the axle he has.

Other than the sway bar mounting brackets present on the Suburban 8.5" axles, are they notably different from the pickup 8.5" axles? I'm assuming the widths are the same too, for C pickup/Suburban or K pickup/Suburban.

I've seen Monroe et al. list different p/n for the shocks, pickup vs. Suburban, but I never researched the difference.

And maybe I shouldn't be saying "Suburban" but rather "Utility", which includes Tahoe and "Blazer" / short wheelbase utility.

 

[Schurkey]

The magazine article that I followed to swap axles on my '88 K1500 put a "Utility" axle under their pickup...or the other way around. They remarked on the sway bar brackets, but I don't remember if they were missing or extra.

 

[Redsled]

you'll

I just noticed this ^^^

Depending on the MC you've used (quick-take-up or not) you'll be better served if you use the corresponding calipers, e.g.,

- quick-take-up MC is used with low-drag calipers typical of JB5/JB6, having 76mm pistons

- "regular" MC (e.g., the MC used on the JB7+ brakes for GMT400 K2500-3500s or the NBS MC, both of which are not quick-take-up designs) is used with "regular" calipers (JB7+) available w/ piston diameters of either 80mm or 86mm.

Thank you to everyone who replied. This has been very helpful. I wish I knew this information sooner!

I have the NBS MC. Thst was the first thing i did after buying the truck. It had a terrible soft pedal. At that time I was unaware there were differences.

That said, for the front then it sound like I should get the 80 or 83mm calipers off a 3500 chassis as noted above. Is it the same prodcedure then? Swap the correct hub/rotor/caliper from a 3500?