Jeep Update v6.3 - JB7 Brake Swap

Yay!Gurrr

Yay!Gurrr

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Mar 17, 2005
Location
N. N. Raleigh, NC
Im running 1tons w/ stock 3-3/8 calipers and 3/4ton calipers out back.. Was running the stock YJ booster w/ '78 Grand Marques Master.. I can stop the jeep pretty well but have to use a lot of the pedal travel to stop when in 4low (automatic/4:1 tcase). I have always lacked enough fluid volume to the rears that they will continue to spin in 2wd on wet ground.. anyway......

My new tow pig has the 8600# package which gives it the JB7 brake package (GMs nomenclature for different size brake systems, 1/2ton=JB5, 3/4ton7200#=JB6, 8600#=JB7, JB8 etc..) Some parts swap between levels some require the next bigger part but they all seem to swap...

So while i had the master off I said hmmmmm... i wonder if that will fit on the heep.. SO i took it off the tow pig and to mu good fortune it was very similer in many ways to the stock YJ unit... stock YJ unit is ~8.5" diameter, the JB7 unit is 11" This will give me the ability to easily handle the large bore masters cylinders. So I bought a reman unit for the tow pig Note: the booster is same as the JB6 reg. 3/4 tons and I could have got one at the junk yard, but that takes time and cost $$ SO i bent over for a remanufactured unit on the tow pig.

The 4 bolt mounting flange was slightly larger and used larger bolts, but was very similar to the YJ unit and only required me to enlarge the stock jeep mounting bracket holes to 1/2". The push rod was exactly the same length. The bolt spacing for the master cylinder was wider but since I was installing a matching newer style master cylinder I didn't care about that. (just wanted to mention it) Also the master mount is NOT recessed like on the YJ unit.. That could make it easier to adapt to other masters.

I needed to tweek the brake pedal as the pedal stud was 1/2" and the new master requires a 9/16"-ish stud. I thin bushing could also work.

Fitment - It just fits :) It clears the hinge mounting rib on the hood but ~3/4" and clears the top of the hood also by 3/4"

Even if it doesn't work, it looks good !!

JB7-Booster-Compare2.jpg

JB7-Brakes1.jpg

JB7-Stock-YJ-bracket.jpg
 
What's the bore on the MC's? I've got one from a Chevy truck and it has no damn markings on it.
 
The old one (mercury grand marquise) was 1-1/8"

Te new JB7 master is 1-1/4" It should help push the volume i need.

What is yours from ?
 
I wasn't happy with the brakes on my cab truck either when I finished the build. I couldn't hold the truck/lock up the tires on this really steep hill in my back yard. I ended up swapping out the stock Ranger 15/16" bore MC for the Marquis 1-1/8" bore version to get the pedal travel back to normal. I also swapped out the stock 8-3/4" booster for a 10-3/4" version off an early 90's F350 for more line pressure. It had the same stud mounting pattern, but hit the wiper motor and steering shaft due to the larger size. I had to space it off the firewall about 2-3" and extend the shaft to connect to the pedal. The brakes are good now.
 
yager said:
The old one (mercury grand marquise) was 1-1/8"

Te new JB7 master is 1-1/4" It should help push the volume i need.

What is yours from ?
Click to expand...

Honestly, I have no idea, all I know is it's a chevy truck from around 1990. I bought it several years ago.
 
I am not running a prop valve at all

not having enough fluid to the rear, but you have an adjustable prop valve, and running dics in the rear

I thought you might be running a stock proportioning valve

people advise not to do this, but I know several people not running one, and they seem to like it


I am just rambling i guess, to many :beer: tonight
 
BTT:

Also a quick update, while finishing the install up, I took some measurements of the jeep pedal hanger vs the truck hanger and I see where some of my previous issues may have been caused. It all boils down to pedal throw. I didn't have a way to compare various jeep parts only my truck and jeep but that in mind I had noticed that the truck had a longer pedal throw and was capable of pushing the master ~1.25" on a full stroke when on the floor. (Master has 1.5" stroke)..

..The jeep floor is angled under the pedals and the foot/pad actually barley clears most of the travel and hit preventing the last .5" of travel to the firewall.. And with any sort of mat it gets in the way. This setup combined to give me not quite 1" of master cylinder.. I also had a sloppy booster rod where it slipped on the pedal. That 1/8" = over .5" pedal height change.. So the smaller master size (bore) and my pedal 'slop' had my pedal not really coming in until was almost on the floor. NOTE: This could very well be caused by my use of a 1977 CJ pedal assembly, Im not sure what the CJ floors looked like for that year...

So I moved the foot pad up 1/2" to clear the tapered foot well on the jeep floor. This and the pin clean up gave me a true, full padal. And this may have very well be 'just' enough to 'fix' the brakes and make them work.. But not good enough for me..

The new larger bore master (1-1/4")definitely was the trick for me. The larger booster definitely doesn't hurt either.

As mentioned previously i did 'move' the pin on the pedal ~.25" up and it did raise my static pedal height, im not sure yet how i like this, but i plan to shim out the booster assembly with washers if needed. Then if that works i can replace my pedal with a stock one
 
what year was the JB7 stuff from? chevy i'm assuming?

edit: from some more digging, i found its from a '95 chevy, 3/4-ton, 2wd/454?? at least that was the most recent tow rig of yours i could find...

thanks again
 
yes you are correct 1995 Chevy C2500, Booster is same for JB6/JB7 master is 1-1/4" for JB7 stuff (not positive whats on the JB6. On the 3500 SRW trucks they used same master w/ Hydroboost, and larger master on DRW also w. hydroboost..
 
Just a quick update, the brakes work great. Stopping is very easy and I can easily hold 4:1 low with the auto with no trouble..

I need to reblead the rears and adjust the prop but thats np..

Also when I did the install I reused the camaro smaller sized vacuum hard line and hose to the booster. If I hit the brakes repeatedly I feels/sounds like its not pulling enough vacuum to the booster for a half sec.. I can still easily stop just a the loud woooshing noises..
 
yager said:
yes you are correct 1995 Chevy C2500, Booster is same for JB6/JB7 master is 1-1/4" for JB7 stuff (not positive whats on the JB6..
Click to expand...

JB6 is 1-1/8" bore from everything I've found.
 
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