JB5 JB6 JB7 JB8 Size Thread

[evilunclegrimace]

JB3 specs; Front disc 11.57 X 1" with 2 1/2" piston caliper
Rear drums 254 mm X 57 mm ( leading/trailing)
Vacuum-- Single Diaphragm
The easy way to identify JB3 brakes is the front rotor is solid not vented. It was also identified as JN3 in the service manuals.

 

[magnumb]

I was going to add this to the top of the thread, but they are not allowing any editing

 

[evilunclegrimace]

I was going to add this to the top of the thread, but they are not allowing any editing

Oh well, there are not as many JB3 equipped trucks out there as there are JB5 and up but I thought it would be good info to add for the folks that have them.

 

[AvgJoe]

I'm curious changing to the JB7 system. I had to change my proportional valve and the '94 valve is obsolete. I ended up getting one for an '88. I've already changed the master to the NBS and hate it. I have deleted the RWAL dump valve. I plan on changing to the 3/4 ton master and calipers in a few weeks. Just worried about the proportional valve. Any insight??

 

[Steve A]

With both regular and low drag calipers in the system, I wonder how many have replaced calipers with a rebuilt/loaded caliper from the parts store and are now running around with a mismatched set of calipers? Would explain a pull to one side that has been otherwise undiagnosable.

 

[mars2878]

def sounds plausible

 

[95C1500]

I'm curious changing to the JB7 system. I had to change my proportional valve and the '94 valve is obsolete. I ended up getting one for an '88. I've already changed the master to the NBS and hate it. I have deleted the RWAL dump valve. I plan on changing to the 3/4 ton master and calipers in a few weeks. Just worried about the proportional valve. Any insight??

Never heard of someone disliking the nbs master cylinder before. Why don't you like it?

 

[shamrock246]

Could never get rid of slop when I first step on the brake pedal it looked to be because it was taking up the gap in the low drag calipers. I'm tempted to try it with 3/4 ton conventional calipers.I like the resovior makes it much easier to use a pressure bleeder. I did notice a little more braking effort but that didn't bother me that much.

 

[AvgJoe]

I only changed the master cylinder. Not the booster. The pedal feel was great but I had to stand on the pedal to get the truck to stop. I deleted the RWAL module but even then, the tires would not lock up. None of them. I did the swap to 3/4 ton calipers but now have the hodge-podge of brake pads. I need 3/4 ton inboard pad with a 1/2 ton outboard since it interferes with the rotor. I put the 1/2 master back on for now until I can determine where to go from here.

 

[1998SSLT]

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

So I got a newbie question. Can these be randomly changed? I've got a JB5 can I put JB6 on rear and leave front alone or am I just stuck with what I've got? Thanks for all your hard work and time on this by the way.