J8 Help
JK Jedi
Joined: Jun 2013
Posts: 11,362
Likes: 2,089
From: Willow Park, TX
Decided to go with the J8 kit. Mopar has been out of the kit for over a year and no resupply in sight. So I found an alternative knock off J8 from Summit and Crown. Part numbers 68048757AA and 68048755AA. DO NOT BUY!!!!! The master cylinder failed 3 times! Each time the brakes worked and then after a week or so the peddle went to the floor and the Jeep would creep forward at a stop. The internal seals had failed each time.
Jeep offers the J8 which is 1 1/8 inch bore (not currently manufacturing), an OE 1 1/16 inch bore and an OE 1 inch bore. I put my stock booster back in and bought the 1 1/16 inch bore master cylinder for the JKU and it works great. Purchased it from the local parts store. Just make sure you get the model with the 1 1/16 inch bore and not the 1 inch bore because both parts show up in their search. It will state the specs on their screen. So much headache could have been avoided.
This is great info and something I might try as well. Guessing you don't have that specific part number huh?
Last edited by resharp001; Sep 24, 2022 at 04:07 AM.
JK Newbie
Joined: Aug 2022
Posts: 3
Likes: 2
From: Vacaville, CA
Brake booster JKU
JK Freak
Joined: Apr 2009
Posts: 775
Likes: 32
From: Colorado
Reading this post from interest. I installed the J8 Mopar BBK Master & Booster kit about this time in 2016, matched up with a Tera front BBK and recently the Tera rear BRK. The Mopar master/booster works well, if a bit more pedal travel than I like. I have gotten a bit concerned after reading the J8 kit is no longer being made by Mopar, and reviews I read regarding the Crown J8 replacement M/C & booster aren’t encouraging. Has anyone had good results from the Crown J8 kit?
JK Newbie
Joined: Feb 2019
Posts: 5
Likes: 0
From: Des Moines, IA
I'm struggling w/ the Crown J8 kit MC (68048757AA part#). This is the 1 1/8" bore. I bench-bled and was super careful putting in after reading stories of bending the pushrod/etc. I did put the (I think) correct o-ring btw the MC and the Booster but perhaps it was wrong?
Problem - brake pedal sinks to the floor and I hear fluid passing by after bleeding correctly - makes me thing a seal is cut on the Crown MC? I'm not losing brake fluid, so I think it's just sneaking by seals in the MC. I'm going to take the MC apart and check for any cut seals. Maybe I messed it up when I bench bled it???
@Warthogjku do you know if the MC 1 1/16 inch bore NMC90072 can be used with the J8 booster (68048755AA part#)? I don't want to take the upgraded booster off (PIA) and not sure I can find (Nor want to) put the Crown J8 kit MC (68048757AA part#) back on. I can't return the J8 kit because I bought in April of '22 (some of my projects take awhile to finish, lol...).
FYI I bought my 2015 JKU wrecked and the drivers caliper had been ripped off - it sat for probably a good 1.5 years until I replaced everything. Never could get good feel from the brakes - replaced the stock MC 2x's then upgraded to the Crown J8 and bigger front rotors/calipers. I thought the J8 MC would resolve the issues. I have another JKU that I'm fixing for my son, so I swapped the ABS pump thinking that might be doing something odd but it's not (I have software that let's me cycle the ABS - it's not air in the system causing the problem).
Problem - brake pedal sinks to the floor and I hear fluid passing by after bleeding correctly - makes me thing a seal is cut on the Crown MC? I'm not losing brake fluid, so I think it's just sneaking by seals in the MC. I'm going to take the MC apart and check for any cut seals. Maybe I messed it up when I bench bled it???
@Warthogjku do you know if the MC 1 1/16 inch bore NMC90072 can be used with the J8 booster (68048755AA part#)? I don't want to take the upgraded booster off (PIA) and not sure I can find (Nor want to) put the Crown J8 kit MC (68048757AA part#) back on. I can't return the J8 kit because I bought in April of '22 (some of my projects take awhile to finish, lol...).
FYI I bought my 2015 JKU wrecked and the drivers caliper had been ripped off - it sat for probably a good 1.5 years until I replaced everything. Never could get good feel from the brakes - replaced the stock MC 2x's then upgraded to the Crown J8 and bigger front rotors/calipers. I thought the J8 MC would resolve the issues. I have another JKU that I'm fixing for my son, so I swapped the ABS pump thinking that might be doing something odd but it's not (I have software that let's me cycle the ABS - it's not air in the system causing the problem).
JK Freak
Joined: Apr 2009
Posts: 775
Likes: 32
From: Colorado
I'm struggling w/ the Crown J8 kit MC (68048757AA part#). This is the 1 1/8" bore. I bench-bled and was super careful putting in after reading stories of bending the pushrod/etc. I did put the (I think) correct o-ring btw the MC and the Booster but perhaps it was wrong?
Problem - brake pedal sinks to the floor and I hear fluid passing by after bleeding correctly - makes me thing a seal is cut on the Crown MC? I'm not losing brake fluid, so I think it's just sneaking by seals in the MC. I'm going to take the MC apart and check for any cut seals. Maybe I messed it up when I bench bled it???
@Warthogjku do you know if the MC 1 1/16 inch bore NMC90072 can be used with the J8 booster (68048755AA part#)? I don't want to take the upgraded booster off (PIA) and not sure I can find (Nor want to) put the Crown J8 kit MC (68048757AA part#) back on. I can't return the J8 kit because I bought in April of '22 (some of my projects take awhile to finish, lol...).
FYI I bought my 2015 JKU wrecked and the drivers caliper had been ripped off - it sat for probably a good 1.5 years until I replaced everything. Never could get good feel from the brakes - replaced the stock MC 2x's then upgraded to the Crown J8 and bigger front rotors/calipers. I thought the J8 MC would resolve the issues. I have another JKU that I'm fixing for my son, so I swapped the ABS pump thinking that might be doing something odd but it's not (I have software that let's me cycle the ABS - it's not air in the system causing the problem).
Problem - brake pedal sinks to the floor and I hear fluid passing by after bleeding correctly - makes me thing a seal is cut on the Crown MC? I'm not losing brake fluid, so I think it's just sneaking by seals in the MC. I'm going to take the MC apart and check for any cut seals. Maybe I messed it up when I bench bled it???
@Warthogjku do you know if the MC 1 1/16 inch bore NMC90072 can be used with the J8 booster (68048755AA part#)? I don't want to take the upgraded booster off (PIA) and not sure I can find (Nor want to) put the Crown J8 kit MC (68048757AA part#) back on. I can't return the J8 kit because I bought in April of '22 (some of my projects take awhile to finish, lol...).
FYI I bought my 2015 JKU wrecked and the drivers caliper had been ripped off - it sat for probably a good 1.5 years until I replaced everything. Never could get good feel from the brakes - replaced the stock MC 2x's then upgraded to the Crown J8 and bigger front rotors/calipers. I thought the J8 MC would resolve the issues. I have another JKU that I'm fixing for my son, so I swapped the ABS pump thinking that might be doing something odd but it's not (I have software that let's me cycle the ABS - it's not air in the system causing the problem).
JK Newbie
Joined: Feb 2019
Posts: 5
Likes: 0
From: Des Moines, IA
Update - ended up buying a Mopar J8 Master Cylinder off of Ebay (real factory one - expensive (>$300 but I need to be able to stop and didn't want to go back to the orginal booster since I had the J8 setup....). Bench bled the MC- made sure to not push all the way to the stop, just about 3/4" back and forth (had plastic fittings to go in the 2 ports w/ hoses on the ends that went back into the 1/2 full Brake reservoir - not hard to do). Installed and felt better right away - bled brakes and cycled the ABS with some software and cable we bought (need to find where my buddy bought but worked great and only cost like $35). Brakes are great now - pedal doesn't sink to the floor. I took apart the Crown Mastercylinder and can't see anything noticeably wrong (Internal seals all look good). All I can guess is the Crown MC has poor manufacturing tolerances and doesn't work. I really don't think bench bleeding it did anything wrong -- pretty easy to bench bleed like I did and I really don't think on a NEW MC that pushing all the way through the entire stroke would hurt it but maybe it did cause the Crown to not work.