JB5 JB6 JB7 JB8 Size Thread

Discussion in 'Axles + Brakes' started by magnumb, Nov 14, 2017 at 10:31 AM.

  1. magnumb

    magnumb Newbie

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    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
     
    Last edited: Nov 14, 2017 at 2:39 PM
  2. magnumb

    magnumb Newbie

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    I'm changing out my JB6 and replacing the front calipers with 3.15" piston and 1.25" master cylinder.
    Leaving the rear wheel cylinder the same for now.
     
  3. shamrock246

    shamrock246 Newbie

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    It looks to be the same info i found,I didn't know the JB8 had a different master than JB7 must have missed that on Rockauto

    Keep us updated i'm looking into the JB7 master cylinder,calipers but leaving it vacuum and if i'm not happy it's ready for hydro boost.
     
  4. 95C1500

    95C1500 ┌П┐(◣_◢)┌П┐

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    I've never heard of JB8. Interesting.

    Good info!
     
  5. magnumb

    magnumb Newbie

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    Will do.
    I see that JB7 Calipers are not "Low Drag". I'm wondering if this will improve pedal feel as the pedal won't have to move as much to make contact with rotor?? This may be negated by the fact i'm moving to a small dia. master cylinder bore which requires more pedal travel though...
     
    Last edited: Nov 15, 2017 at 9:56 AM
  6. chipskittles

    chipskittles I'm Awesome

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    Low drag calipers? I'd never come across this but I found the following that might interest someone else as well. No easy way to tell what calipers you have but master cylinders are pretty clearly labeled as stepped or not if you know what to look for. All our trucks are well beyond the expectation that they will have original parts at this point so it's just something to be aware of...

    LOW OR EXCESSIVE PEDAL TRAVEL CAUSED BY THE LOW DRAG CALIPER

    What is a low drag caliper? In 1980 GM introduced the low drag caliper on its vehicles. The reason for the low drag caliper was the energy crisis. Low drag calipers were designed to reduce the rolling resistance of their vehicles thereby giving you better gas mileage. This rolling resistance is produced by the friction generated by the pads rubbing against the rotor when the brakes are not applied. The engineering people at GM found that a change in the square cut seal groove on the caliper could cause the caliper piston to be pulled back twice as much as the conventional caliper. The low drag caliper uses a 30 degree bevel in the seal groove as opposed to the conventional calipers' 15 degree bevel. Twice the bevel causes twice the seal flex. Flexing the seal twice as much pulls the caliper piston back into the caliper bore twice as far...hence no drag with the low drag caliper.

    Whats the problem with using a low drag caliper?
    A conventional master cylinder will not work with a low drag caliper. The volume of fluid needed to take up the extra gap created by the low drag caliper exceeds the conventional master cylinder's capacity. To fix this problem GM designed whats called a quick take up master. These cylinders are of a stepped bore design. They provide a large volume of fluid initially to take up the gap and then complicated internal valving switches the master over to a high pressure system where it acts as a conventional master cylinder. If you don’t use a quick take up master with low drag calipers you will never get a good pedal

    How do you visually check if your calipers are low drag?
    Unfortunately low drag calipers are mixed in throughout the system with conventional calipers, especially if rebuilt calipers are used. Externally there’s no way to tell. One helpful hint is that the majority of low drag calipers were of the smaller GM design with a caliper mounting bolt spacing of 5-1/2” as opposed to the large GM caliper with a 7” bolt spacing.

    How do you physically check if your calipers are low drag?
    To check if your caliper is low drag perform the following test. Obtain a pair of brake hose clamps at an auto parts store. Try the pedal as it is and then clamp off the two front hoses. If the pedal returns and is high and firm chances are you have low drag calipers. Be aware that this same sympton will occur if there is still air in the caliper or the bleeder screw that lets the air out during bleeding is not directly on top in the 12:00 position facing up.
     
    Last edited: Nov 15, 2017 at 11:29 AM
  7. magnumb

    magnumb Newbie

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    Nice find!
    I guess moving to the JB7 3.15" saves you the guess work, but will also require a different master. How do you tell if a master is stepped bore? I'm guessing the 1.25" masters are not.

    I also couldn't find anything stating the the 3.15" calipers also incorporated low drag seals....which is good for pedal feel.
     
  8. chipskittles

    chipskittles I'm Awesome

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    The thing that will help tie it together is if the master cylinder bore lists two values(stepped). Never have I owned vehicles that had so many variations of parts. My wife's Avalanche is the same way lol
     
  9. magnumb

    magnumb Newbie

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    Just recvd the JB7 master cylinder with 1.25" bore and it doesn't look to have the quick uptake section.
    I'm thinking this along with the 3.15" calipers will be the best setup for the money on a 1/2 ton with JB6 RPO code. Total cost was $73. 2 calipers from AutoZone and Cardone 13-2684 MS from Amazon.
    Installing this weekend and will post results.
     
  10. shamrock246

    shamrock246 Newbie

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    The quick take up master was used on the JB5 and JB6 brakes with the 75mm dia piston calipers (low drag). The master for the JD6,JB7,JD7,jd8 are all the same Vacuum or Hydroboost and should work fine with the 80mm 3/4 ton caliper those calipers should be convention calipers i think the JD6 system used the low drag 75mm calipers . The whole low drag calipers and quick take up master has been used on chevy stuff since the early 80's my 83 caprice wagon had it but the brakes were very good and would throw you thru the wind shield if you looked at the pedal the wrong way not sure why it didn't work out on these trucks.
     

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