JohnFin

I installed UD60's in my '14 Rubi and upgraded the master cylinder and power booster to a J8 setup. I've gravity bled the brakes, two person bleed the brakes, pneumatic bled the brakes and afterwards the pedal would always go to the floor. I'm not a mechanic, but I've been doing my own brakes for almost 40 years. I couldn't figure it out, so I took it to two shops who specialize in brakes, both shops ABS bled and power bled the brakes, and still the brake pedal would go to the floor. It was finally determined that I had a defective master cylinder. I installed another master cylinder, bled the brakes and all was good for about a week, now I'm back to the same problem, the pedal goes to the floor about every 4th time I stop the vehicle. If it were air in the lines, wouldn't the brake go to the floor every time? I'm beginning to think it's a power booster or vacuum issue....
Any advice would be very helpful, thanks. BTW, I've had 2 YJ's, 1 TJ, and still have 2 JK's and a WK2, not a "newbie", but sure feel like one with this situation.

resharp001

I bought this setup when it became available again. The Mopar MC that came in the kit was defective. I bought this through a friend who has a shop, not through a retailer, so I knew I didn't have the same sort of replacement warranty without some major headache which would include driving way across town several times. Anyhow, I purchased a new Crown MC as a replacement and installed it. About a week later the seal on that failed. I got a replacement MC and uninstalled the booster, going back to factory. Too much headache to deal with anymore. I'm using factory booster/MC ('12+) paired with a Crown BBK and it has been ok for me.

JohnFin

I'm about to that same point, just putting the original MC and power booster back on. I've got a Crown version of the J8 MC with the Mopar J8 power booster, like I said, it worked great for about a week and now it's pedal to the floor about every 4th time I stop, which is what's confusing me, if it's the MC or air in the lines, it should go to the floor every time......

resharp001

When I bought the Mopar J8 MC/Booster kit, the first thing I noticed is that MC was much longer than the upgraded MC that came with the old Mopar BBK. It was very tight getting it installed in my '13. I don't know if in the process of trying to get it in there i got the post from the booster a bit cockeyed in the MC or what, but the piston seized pretty quickly when testing it after install. That is when I bought the Crown MC as a replacement. I bench bled it, installed it, and it was fine for a few drives. Then, I noticed the same things you are noticing. Removed it and sure enough, losing brake fluid into the booster. I just went back to the factory and figured if I wasn't happy after a bit, I'd look at the TF MC. It seemed most that I read suggested that the '12+ MCs were fine to use with TF's BBK, but the pre-'12's would need the upgrade. It's been fine for me so I just left things as/is. I don't know if I just got a crap Crown MC or if there is a problem with them. About the same time that I installed that Crown BBK, there was another member on here that did it as well and used the Crown J8 booster and MC with no issues at all. Starting to think that you having been through 2 Crown MC's, that there is an issue. Either the seals in those are crap or that booster is just too much. That sure doesn't help anything, but take a bit of solace in the fact you're not alone.

JohnFin

Update: I'm on the 4th master cylinder and 2nd power booster and finally have awesome braking. I was on the verge of giving up on the J8 setup, but after the last installation, the brakes are performing terrific, and much better than oem. I knew it wasn't anything I was installing incorrectly or lack of proper brake bleeding, it was either a defective master cylinder or defective power booster. I just hope what's installed now doesn't fail any time soon, although after all of the swapping in new brake parts, I could replace them blindfolded now.

resharp001

Pretty crazy it's taken so much. rear seal blew on all three of the previous MCs? Do you have a thought what was wrong with the original booster? You think the booster itself was leading to the MC failures?

JohnFin

I'm thinking it was actually the seal between the mc and booster. The first kit I got didn't have a seal with it and I was directed by the supplier to use the seal off of my oem mc. After several attempts at a different mc, they just sent me a whole new kit, this time it had a seal with the kit. So most likely the mc's and power boosters were good and I should have received a new seal with the first kit I got.

resharp001

You're talking the seal that goes around the back of the MC as it inserts into the booster right? if i recall, the Mopar MC that came with my kit had a seal, but when I purchased a replacement Crown MC it did not come with a new seal so I just used the one from the Mopar MC. IDK, Given how quickly the Crown MC I had failed I almost think there's something with the booster itself that was just too much for the rear seal in the MC. Glad you finally got it working. Please follow up though if you run across another failure. cross your fingers.....

JohnFin

Yes, the seal that goes between the mc and booster. I compared the new seal that came with the last kit I got to the oem seal that I was told to use and the new seal is just a hair bigger in diameter, but at least twice as thick. So I'm guessing it's been the seal this whole time.....

Warthogjku

I built 1 tons for my 2012 JKU automatic. 2010 Ford SD60 front and 14 bolt rear with stock Ford and GM brakes. Running 40 inch tires. Had my stock master cylinder and brake booster for a month. They were spongy but stopped the Jeep. 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. This was after multiple attempts to bleed using various methods and wracking my brain. Finally returned the whole set up to Summit after so much frustration. 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.