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JB6 or JB7 brakes ?

Discussion in 'The Garage' started by 77crewcab, Aug 1, 2004.

  1. 77crewcab

    77crewcab 1/2 ton status

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    I need to get a new master cylinder and booster and don't know whick brake code I have. The sticker in the glove box doesn't say so how can I tell whick brake code I have? The truck is a 77 C20 crewcab.
    Thanks guys!
     
  2. ugly_blazer

    ugly_blazer 1/2 ton status

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    I had the same problem on my K20. The local napa ordered both and I could not tell the difference between them. The JB7 is rated for a higher gross vehicle weight so I went with that one. The only difference I have noticed in the 3/4 ton braking systems is the size of the rear drums, the 2 wheel drive trucks sometimes had smaller diameter ones.
     
  3. u2slow

    u2slow 1/2 ton status

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    From here:
    http://tlentz.oldsgmail.com/rpotext.txt

    JB1 BRAKE, MANUAL, DISC/DRUM, 4300 LBS
    JB3 BRAKE, POWER, DISC/DRUM, 4600 LBS
    JB5 BRAKE, POWER, DISC/DRUM, 6400 LBS
    JB6 BRAKE, POWER, DISC, DRUM 7200 LBS
    JB7 BRAKE, POWER, DISC, DRUM 8400 L85
    JB8 8RAKE, POWER, DISC, DRUM 10000 LBS

    EDIT: C20 crewcabs are built on a 1-ton frame. I'd lean toward the heavier JB7 brakes. My Dad's '79 C20 crewcab had 8200lb GVW, and JB7 brakes.
     
  4. 77crewcab

    77crewcab 1/2 ton status

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    Thanks that helps. My truck has the 8200GVW so the JB7 is what I need then. But does that mean that I could upgrade to the JB8 if I wanted to have more braking or is that for the dually?
     
  5. dremu

    dremu Officious Thread Derailer Premium Member

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    Greets fellow /forums/images/graemlins/crewcab.gif owner! There's not much info here for these trucks, either 'cuz it's a /forums/images/graemlins/k5.gif site, or cuz there just aren't many sixpacks out there. /forums/images/graemlins/smile.gif

    Anyway, I'm fiddling about with the brakes on my new '74 and here's what I know:

    JB8 is the heavy duty one-ton brakes, 9000-10000 GVWR. It has a master cylinder with a 1 5/16" bore.

    JB7 is the light-duty one-ton and heavy-duty 3/4-ton brakes, 6600-8200 GVWR. Master here has a 1 1/4" bore.

    JB6 looks like the light-duty 3/4 ton and heavy-duty 1/2 ton, with a 1 1/8" bore master.

    Note that when switching masters, you gotta switch boosters to match.

    I'm thinking that when it comes time to replace the master and booster on mine (also a C20), I'm gonna switch to the larger-bore (JB8, 1-ton HD, whatever /forums/images/graemlins/smile.gif)

    -- A
     
  6. 77crewcab

    77crewcab 1/2 ton status

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    So without the rpo codes in the glovebox how can I tell for sure which I have? Does it depend on the rear drum size? Also since you have a CC can you check something for me? On your door plate does it have something about camper weight ratings and a hole inside the bed above where the filler neck is. I am wondering if maybe mine had the camper special package and maybe the camper wiring has been taken out. I have nine springs in the rear and it seems to sit pretty high for a 2WD both front and rear. Unfortunately my sticker in the glovebox list very few of the rpo codes for the truck and has mainly simple stuff like the color and mirror style and such so I am at a loss for a lot of things.
     
  7. u2slow

    u2slow 1/2 ton status

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    [ QUOTE ]
    But does that mean that I could upgrade to the JB8 if I wanted to have more braking or is that for the dually?

    [/ QUOTE ]

    I did JB8 brakes on my Suburban. You need 1-ton spindles, lower balljoints, SRW rotors, calipers, pads, & flex hoses to do the front. Rear needs 1-ton (13x3.5") backing plates, SRW drums, & shoes.

    1-ton boosters are all hydroboost, so you need to monkey with your ps pump & lines to convert from a vacuum booster. You may be able to use the 1-ton master on the vacuum booster... I'm using a 3/4-ton master on a 1-ton booster because it was handy at the time.
     
  8. dremu

    dremu Officious Thread Derailer Premium Member

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    [ QUOTE ]
    So without the rpo codes in the glovebox how can I tell for sure which I have? Does it depend on the rear drum size?

    [/ QUOTE ]

    I believe so. 11 5/32" shoes are 2 3/4" wide, which should be JB6.

    13" shoes are 2 1/2" wide, which should be JB7.

    As previously discussed, JB8 might well be hydroboost and is way complicated, so you'd know if you had that (plus that should only be on one-tons.)

    [ QUOTE ]

    Also since you have a CC can you check something for me? On your door plate does it have something about camper weight ratings and a hole inside the bed above where the filler neck is. I am wondering if maybe mine had the camper special package and maybe the camper wiring has been taken out.

    [/ QUOTE ]

    Mine was badly molested by a PO and now has an early 80's cab on it. Sorry /forums/images/graemlins/frown.gif /forums/images/graemlins/doah.gif I won't even try and describe the evil things that have been done to it, except to say that I'm pretty sure I saved it from a fate worse than death (lowering it and making it blingy /forums/images/graemlins/smile.gif)

    [ QUOTE ]

    I have nine springs in the rear and it seems to sit pretty high for a 2WD both front and rear. Unfortunately my sticker in the glovebox list very few of the rpo codes for the truck and has mainly simple stuff like the color and mirror style and such so I am at a loss for a lot of things.

    [/ QUOTE ]

    Could have had an add-a-leaf added; I'll hafta count mine when it's handy. The RPO sheet is usually pretty comprehensive. You might check out the Trailering Guide for 1977 on my Brochures page (shameless self-plug there /forums/images/graemlins/smile.gif) which lists a bunch of options and RPO's. Also check out page 13 of the Pickups papmhlet, which implies that Camper Special trucks had a badge for same (on six-packs, I thought it replaced the 3+3 logo aft of the rear door.)

    HTH.
    -- A
     
  9. jarheadk5

    jarheadk5 1/2 ton status

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    [ QUOTE ]
    As previously discussed, JB8 might well be hydroboost and is way complicated, so you'd know if you had that (plus that should only be on one-tons.)

    [/ QUOTE ]

    Hydroboost was on diesels too, regardless of class. The '87 C10 I'm buying as a 6.2 donor has Hydroboost.
     
  10. diesel4me

    diesel4me 1 ton status Premium Member

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    If you call your local chevy truck dealer parts department and give them the VIN number off your truck,their computer will tell you what parts your truck was assembled with--at least it works that way on the 88 and up trucks,I'm not sure how far back the database goes.If they cant help you,then your going to have to measure the drums in the rear,and the front rotors,maybe take a wheel cylinder apart and read the bore size on the cups inside.Just guessing you probably have the JB7 brake package,but since so many things interchange on a chevy truck,who knows for sure--anything could be on there by now!. /forums/images/graemlins/crazy.gif
     
  11. u2slow

    u2slow 1/2 ton status

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    [ QUOTE ]
    Also check out page 13 of the Pickups papmhlet, which implies that Camper Special trucks had a badge for same (on six-packs, I thought it replaced the 3+3 logo aft of the rear door.)


    [/ QUOTE ]

    My Dad's '79 came with the "3+3" badge, "Camper Special 8200lb GVW", and the yellow teepee emblem. /forums/images/graemlins/smile.gif
     
  12. 77crewcab

    77crewcab 1/2 ton status

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    Well I am assuming that this truck has the camper special package. Heavy Duty rear springs with 9leafs, a hole inside the bed above the driver side gas tank that looks like it may have been to hook up to a slide in camper and the GVW plate on the door frame has some sort of dimensions for a camper. Anyway I thought it was pretty odd that the parts store wanted $10 more for the master/booster combo rather than buying them seperate.

    One more question, what is the best way to bleed the brakes. I would like to get all the old fluid out of the lines and put in fresh fluid all the way. I've never replaced the mc/booster just calipers so don't know what the best way is.
     
  13. dremu

    dremu Officious Thread Derailer Premium Member

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    S'pose you could put an air gun up to the lines at the master and blow them out... seems a bit hardcore to me.

    There was a thread just the other day about do-it-yourself power bleeding... basically a garden sprayer from your local hardware store. When I do mine I think I'm gonna go that route, or maybe use a spare regulator and some plumbing to my air compressor.

    -- A
     
  14. mudsup

    mudsup Newbie

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    Brakes

    Hey Guy's
    Not sure if this will or if it is too late to help.
    I have a 86 2500 and on the reg it show's T/C for body style and was told it is for camper even though I have no wires for it, and not many rpo's {Bare bones truck}. As for the brakes. Measure the width 2.5 X 13 Should be the heavy duty, Just picked up a 87 2500 4x4 and in the process of looking up the rpo's,
    The JB7 is the 8400 lb heavy duty brakes on the dana 60 and they measure out to that spec.
    Hope it help's
     

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