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What is the Difference, JB5, JB6 and JB7?

Discussion in 'The Garage' started by tch777, Oct 30, 2005.

  1. tch777

    tch777 1/2 ton status

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    I found out I have a JB5 brake system in my :burb: and I have always thought its brakes were kinda weak, even after replacement and going to a shop for power bleed and adjustment.

    I know the ratings for the different brakes but what hardware is different between these systems:
    JB5 BRAKE, POWER, DISC/DRUM, 6400 LBS
    JB6 BRAKE, POWER, DISC, DRUM 7200 LBS
    JB7 BRAKE, POWER, DISC, DRUM 8400 L85
    For example what can I install / replace in my JB5 system to make it a JB7?

    Thanks

    Tim
     
  2. sandawgk5

    sandawgk5 3/4 ton status

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    According to the Chevrolet 10-30 series light truck service manual:

    JB5= front disc 11.86X1.29 rear drum 11.15X2.75 Vacuum-Double Diaphragm

    JB7= front disc 12.50X1.54 rear drum 13.00X2.50 Vacuum-Double Diaphragm

    JB8= front disc 12.50X1.54 rear drum 13.00X3.50 Hydraulic-Hydro-Boost

    JB7 is basically Dana 60 Front disc with the smaller 14FF drum brakes with a vacuum booster and JB8 is Dana 60 front disc with the Large 14FF drum brakes with Hydro-boost.

    Ira
     
  3. BigCountryx

    BigCountryx 1/2 ton status

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    from what I can tell, you should be able to swap in a booster and master from a JB7 truck to replace the JB5 system. Just The masters alone are not interchangeable, but with the boosters should be. The master for the JB7 system is also the same master used in 1/2 and 3/4 ton hydro systems from what I can tell.
     
  4. tch777

    tch777 1/2 ton status

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    Can anyone verify applications or which vehicles are donor vehicles and what all I need to swap?

    Thanks
     
  5. Blazer X

    Blazer X Registered Member

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    When I converted to 1ton I went to autozone and they cross-referenced all of the different styles of JB applications. My donor vehicle was a diesel but the hydro boost was trashed so I went with the K20 booster and master and all is good. :laugh:
     
  6. one ton burb

    one ton burb 1/2 ton status

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    I swapped to a jb7 master cylinder with out changing booster mostly because i don't have the jb7 if i did i would change it right away for the proper one, now not trying to steel or hijack this post but right after i swaped to the jb7 booster on my burb it seems like i have little to no front brakes but i have so much rear brakes that i can skid my 33's on dry pavement at 60 with no problems, so any ideas if this is because of the brake bootser being from a jb5 or is it something else.
     
    Last edited: Nov 1, 2005
  7. K85 Octane

    K85 Octane Tacklin' Fuel Premium Member GMOTM Winner

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    Using an old thread.

    What do Blazers come with? I'm curious if I have the JB5 or JB7 setup. But here's the catch, my rears are disc and I'm wondering if they are the larger calipers. I think they are too large for the brake lines, meaning I can't feed the larger pistons with enough pressure before my pedal bottoms out.

    Here are some pictures. I've done all kinds of stuff to the system. But I'll start a new thread for any diagnosing. Just wondering what I have. Thanks

    Stock booster with vette master.

    [​IMG]







    Rear calipers. If it's even possible to tell from a picture.

    [​IMG]
     
  8. YZEATER

    YZEATER 1/2 ton status Premium Member

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    the difference is the size of the cylinder in the master cylinder. get the biggest one you can find. you should be able to search for my old posts and find the part #.
     
  9. Blue85

    Blue85 Troll Premium Member

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    If your pedal bottoms out, it is because of volume, not pressure. Having 4 big calipers in the system requires a lot of fluid flow to engage the brakes and the MC is barely able to provide it. Swapping to a bigger MC piston will give you more volume and therefore a higher pedal, but it also increases the pedal effort. Chances are it will be a good swap because MC piston size should logically increase with caliper piston size for the same sort of effort/travel. If you ended up with too much pedal effort, the only option then is to go hydro because that booster gives more boost. You could also pick some intermediate size MC.

    You can also decrease pedal travel by using the brake pedal from a hydroboost truck (again, increasing pedal effort). But the MC is stroking just as much. When the MC bottoms out, that's it - no amount of effort will apply more brakes.

    My experience in a nutshell, FWIW: swapping to rear discs gave a ton of pedal travel and a "whooshing" MC. Tried a replacement master and then a cadillac disc/disc master. All sucked. Never tried a 1-ton MC with the vacuum booster. Converted to hydroboost and then the pedal travel, effort and braking power were all great. That means a bigger MC, different booster and diffrent pedal geometry all at once, so I can't comment on how each piece would have done by itself.
     
  10. nvrenuf

    nvrenuf Bazinga! Premium Member

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    Get a bigger master cylinder.

    On my k5 I'm running the stock vac booster with a '99 C3500 master cylinder and 4 D60 calipers. I have great pedal feel and stopping power.
     
  11. K85 Octane

    K85 Octane Tacklin' Fuel Premium Member GMOTM Winner

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    Ok haha, the hell with it. I've got a braking problem and I need some help fixing it. On the street I don't consider it dangerous, not in the least. BUT, when I'm in 4L I have trouble stopping while on obstacles. The simplest example is this picture. I put it in reverse and could not stop the truck until it came off the post. :(

    [​IMG]



    Image how I feel nose light, reversing off an obstacle, and kicking it into neutral to help stop. :eek1:


    [​IMG]





    Here are some quick pictures of whats going on. I'm running the stock booster but with a more sensitive reaction valve. At one point pedal was rock hard with different MC, tried vacuum canister. The current MC is from a Vette. I'm running an adjustable prop valve to the rear but it's turned all the way open. No factory prop valve, fronts are straight off MC. Also running 2lb check valves front and rear. D60 front, 14b rear with discs.

    [​IMG]




    [​IMG]
     
  12. K85 Octane

    K85 Octane Tacklin' Fuel Premium Member GMOTM Winner

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    So here goes.

    I tried both a '99 master cylinder and a same year '85 K30 MC. For the life of me I can't remember which did what. One of them gave me a soggy pedal, even after bench bleeding and several rounds of pressure bleeding. Changed to the next MC and it gave me a rock hard pedal, very little travel, and very poor braking. It felt like I was stopping with stones in the calipers and couldn't push any harder. This is when the vacuum canister came onto the scene. Kinda no liking that. I pulled the lines off the MC and the pedal went to the floor, so it wasn't a piston stroke problem.

    ummm..... oh yeah, vacuum is 13-15
     
  13. nvrenuf

    nvrenuf Bazinga! Premium Member

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    What trans? Reduction box of any kind? Axle gears?

    There is a point where the reduction can over come the brakes.
     
  14. K85 Octane

    K85 Octane Tacklin' Fuel Premium Member GMOTM Winner

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    So the current set up is the best it's ever been :(

    I'd rather not go hydroboost, that is a BIG can of worms. I also wouldn't want to tax my hydro assist system. Lastly, I don't like that my brakes solely rely on a mechanical item, rather than simple vacuum from an engine. When my dads duramax had pump issues, driving that to the shop was not fun. I can see losing steering, but brakes too, ooooooooooo man.
     
  15. K85 Octane

    K85 Octane Tacklin' Fuel Premium Member GMOTM Winner

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    I wish. 700/241 4.88s Not enough to over drive the brakes :(
     
  16. Blue85

    Blue85 Troll Premium Member

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    Why not just try the late 80's hydroboost MC, but keep your vacuum booster.
     
  17. K85 Octane

    K85 Octane Tacklin' Fuel Premium Member GMOTM Winner

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    Is that the game I've gotta play now? Try different MCs until I find one that works? How about line size? I'm confused at how troublesome this has become. I know it's re-engineering an engineered system, but most folks don't seem to have that much trouble.

    Is the 80s hydroboost MC any different than the 85 k30 MC I've tried?
     
  18. diesel4me

    diesel4me 1 ton status Premium Member

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    Whoever posted about having the rear brakes lock up too easily ,but seems to have poor front braking,I'd make sure the two lines going to the master cylinder are in the correct ports...

    I swapped a master cylinder off a gas powered 3/4 ton I had onto my diesel trucks hydroboost,it was identical as far as the depth of the pushrod seat in the piston in the MC,and it bolted up fine,but it had two equal sized fluid chambers (Delco style) where as the original diesel one was a Bendix style with a small fluid chamber for the rear brakes and a larger one for the front discs...

    Turned out the steel lines would not screw into the ports where they were originally,but would fit if I swapped them around--this resulted in severe rear wheel lockup and barely any front braking...very scary ride to say the least!..:eek1:..

    I bought two brass fittings to allow the lines to be put back where they belonged,and all is well again...(except the master is leaking out of the back end,just like the one I replaced was again!)..I dont know if the piston sizes in the two MC are different,but I have not noticed any loss of braking if they are on the street..
     
  19. ChrisPerry

    ChrisPerry Dreaming of links Premium Member GMOTM Winner

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    I've tried several different MC on mine, the one I like the best is the k30. I got one off of a 77? I think. I had issues with the soggy pedal. It was bench bled, vacuum bled, pump and purged, ect. with no improvement. Finally I gravity bled it for 30-45 min, using almost a quart of fluid. After that I had awsome brakes. If I remember correctly I did have to switch the lines front to back.

    If you're ever going to be up in Ventura on a Thursday or Friday give me a PM you can try my brakes out.
     
  20. K85 Octane

    K85 Octane Tacklin' Fuel Premium Member GMOTM Winner

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    Hey Chris, would you be willing to look at my clusterf**k? :D

    To address recent comments. My rear brakes will only lock up if I slam HARD on the pedal.
    My rear line is smaller than the front. When on flat ground, in 4L, I've only got ~1/2" of pedal movement till it stops (when I'm on the brakes) if I'm not on the pedal hard, it will want to move.
     

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