JB6 to JB7 brake upgrade

[Idaho_z71]

So I have a 98 k1500 with a 6 lug 14 bolt rear end and JB6 brakes. I'm looking at switching to the JB7 calipers (pn: 18-4123) and the JB7 master cylinder. (pn: 131-66008) My original plan was to switch to rear disks but @Schurkey made some good points in a couple different threads about those conversations and I ditched that idea pretty quick. I guess my question is should I swap my drum brakes or can I run my oem 11" JB6 drums? Also do I need to do anything with the pressure valve (not sure if thats the rught term) for the drums since im going from a low drag JB6 set up, to a non-low drag JB7 set up. I've done a lot of reading on this site and I believe I don't have to change my drums because there doesn't seem to be any big changes from the JB6 to JB7 drums considering they are both duo-servos. but I can't quite get the solid answer I'm looking for. Part numbers are a big help too!

 

[1998_K1500_Sub]

I guess my question is should I swap my drum brakes or can I run my oem 11" JB6 drums? My original plan was to switch to rear disks but @Schurkey made some good points.

I've got a 1998 K1500 Suburban, approximately 3000# heavy on the front and 3200# heavy on the rear, JB6 brakes from the factory but with many changes (see my sig).

Regarding the rear brakes, I have the 11" drums and I swapped out the stock 1" diameter cylinders with the JB8 1.1875" diameter cylinders, and am pleased with the result.

I did not change the combination valve at the MC or make other changes to accommodate the JB8 slave cylinders. I do have the NBS MC but that's not a requirement for the JB8 slave rear cylinders.

The rear disc conversion kits... many appear to be poorly designed. If one was to embark on a disc conversion, an OE factory retrofit (e.g., from a later truck) would be the proper approach. For now, try the 11" drums with the JB8 cylinders, and you may be able to find better friction / shoes for the rear too if you wish. Later, if you're not satisfied, consider an OE rear end w/ rear disc swap.

Regarding the front brakes, the JB7 MC and front caliper change is as simple as it sounds... swap the parts.

I also replaced all the brake hoses with SS. Since you're swapping parts now, I suggest you do that as well.

My $0.02

If you want part numbers, send me a PM and I’ll look up exactly what I bought.

One added comment: If you install the NBS MC now, you're ready to upgrade to rear discs if the desire moves you as it's got the large fluid reservoir.

See also this thread, practically every topic you need to consider is being covered there.

Brakes for towing

I was driving the 92 k2500 doing a dump run going down hill with a load in the back of bed (no trailer) and the truck did not want to stop when I wanted it to. I plan on doing some towing with this truck (dump trailer and possibly short runs with a compact skid steer). So where should I start...

www.gmt400.com

 

[Schurkey]

The JB7 master cylinder has a larger bore than the JB6. Using the '7 master with the '7 calipers makes sense, but it's going to cause a loss of hydraulic advantage on the rear brakes.

The JB7 rear drums are duo-servo, all right, but they're much bigger. 11.x" diameter vs 13" diameter.

I don't know about the wheel cylinder size difference (if any) between the 11.x" drums and the 13" drums.

The point of all this is that you're going to have really-strong front brakes compared to the rear brakes. Better than the other way around, but still unbalanced.

 

[1998_K1500_Sub]

Changing-out the OE JB6 1" diameter rear cylinders for the JB8 1.1875" diameter cylinders (ACDELCO 18E1362, OE for 13" DRW), and leaving the drum brakes otherwise unchanged, may arguably provide a "boost" equal to the ratio of the cylinder areas, i.e., ~141%.

For less "boost", use the OE rear cylinders for 13" SRW, ACDELCO 18E112 1.0625" diameter, yielding a ratio of areas ~113%.

Reportedly these rear wheel cylinders are bolt-in replacements for the JB6 11" OE cylinders. The ACDELCO 18E1362 was, for me.


FWIW, the other parts I used:

- JB7 calipers ACDELCO 18R741 and ACDELCO 18FR742

- NBS MC - ACDelco 18M1159 (there seems to be a bore on this of 1.339" so it's said)


(edit) From what I've seen just by looking around tonight, it appears the JB6 vacuum boost MC has a bore of 1.125" and the JB6 hydroboost MC has a bore of 1.25", but I do NOT trust these numbers (yet). @Schurkey, do you have bore sizes for various MCs / boost combinations?

 

[Schurkey]

From what I've seen just by looking around tonight, it appears the JB6 vacuum boost MC has a bore of 1.125" and the JB6 hydroboost MC has a bore of 1.25", but I do NOT trust these numbers (yet). @Schurkey, do you have bore sizes for various MCs / boost combinations?

No. Most any sizing info I have is by trolling parts-store web sites. I sent a nastygram to NAPA requesting that they provide cylinder/piston diameters for master cylinders, calipers, wheel cylinders...but I don't expect them to add that info to the part numbers that are missing it.

I would be surprised if the booster style caused a difference in the master cylinder piston diameter...but I've been wrong before. I'd expect JB/JD/JN5 or 6 to have the metric equivalent to 1 1/8 bore (and 40mm for the third, Quick Take-Up chamber.) My JD7 uses a 1.25 bore, though.

 

[1998_K1500_Sub]

I would be surprised if the booster style caused a difference in the master cylinder piston diameter...but I've been wrong before. I'd expect JB/JD/JN5 or 6 to have the metric equivalent to 1 1/8 bore (and 40mm for the third, Quick Take-Up chamber.) My JD7 uses a 1.25 bore, though.

You've piqued my curiosity regarding the bore of the MC, and slave cylinders in the calipers and rear cylinders, for the OE systems, with regard to how some of us here including me propose "upgrades" by swapping one for another.

The bits of information I posted in my prior post I gleaned from RockAuto's site, based upon description given in the MC's description and in the specs, e.g.,

- for ACDELCO 18M712 it says "w/ Hydraulic Booster" and 1.25" bore

- for ACDELCO 18M1492 it says "with Vacuum Booster; 11-5/32" X 2-3/4" Brake Shoe" and 40mm bore (only the quick-take-up bore is mentioned, not the "working" bore size as I'll call it)

- for CENTRIC 13066029 it says "11" Rear Drum; 6 Lug" and 1.125" bore; no mention of the booster type

- for CENTRIC 13066030 MC that says "13" drum 8-lug" w/ 1.25" bore; no mention of the booster type

Other mfrs' descriptions aren't very complete.

However, I've not found (yet... I haven't been thorough) any MC for 1500/2500 in my casual review that has other than 1.125" or 1.25" "working" bore size. I'll keep looking.

Meanwhile, many others (including myself) have spoken positively about the "NBS" MC. I've used a NBS MC for a 2001 Silverado and on RockAuto's site for that truck they show:

- ACDelco 18M1159, "for vacuum booster" with bore 1.338", i.e., 34mm; I used this MC
- ACDelco 18M1107 "for w/ Hydraulic Booster" with bore 1.4566929134", i.e., 37mm

Either of these "NBS" MCs has markedly larger bores than those I've noted for the OBS.

This is all the time I have for this, for now.

 

[Supercharged111]

The JB7 master cylinder has a larger bore than the JB6. Using the '7 master with the '7 calipers makes sense, but it's going to cause a loss of hydraulic advantage on the rear brakes.

The JB7 rear drums are duo-servo, all right, but they're much bigger. 11.x" diameter vs 13" diameter.

I don't know about the wheel cylinder size difference (if any) between the 11.x" drums and the 13" drums.

The point of all this is that you're going to have really-strong front brakes compared to the rear brakes. Better than the other way around, but still unbalanced.

This is how my 1500 is set up, but with hydroboost. JB7 master and calipers but the 11" rear drums. The first brake test I did after swapping the master with hydroboost at the same time (prior was NBS MC, JB7 calipers, and the 11" rear drums) I got the rears to lock up before the fronts. It's not done that since. I do plan to get some powerstop rear bonded semi metallic shoes on and tweak my front compound from there to achieve my desired balance. Before the hydroboost, balance was a moot point as there just wasn't enough force to lock anything. Even with the fresh, crappy Napa semi-metallic pads up front, I had a panic braking scenario recently where the fronts started barking but the rears did not so my balance is still not where I want it. I can handle a bit of the back trying to pass the front as that just means stronger yet braking with the trailer hooked up. Don't want to go too far though as it spends most of its time empty.

 

[1998_K1500_Sub]

This is how my 1500 is set up, but with hydroboost. JB7 master and calipers but the 11" rear drums.

Which cylinders on the 11” drums? OE 1” diameter or other?

 

[Supercharged111]

Which cylinders on the 11” drums? OE 1” diameter or other?

OE cylinders, I'm unaware of alternate options. I assume they were given something to weaken the rears since they came in 6 lug trucks. Only hydroboosted 8 lugs seem to lock the rears before the fronts from the factory in my experience.

 

[1998_K1500_Sub]

OE cylinders, I'm unaware of alternate options.

Presumably you’re using the OE 1” cylinders.

See my prior post in this thread:

Post in thread 'JB6 to JB7 brake upgrade'
https://www.gmt400.com/threads/jb6-to-jb7-brake-upgrade.58607/post-1248959