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Unread 09-17-2013, 09:36 PM   #1
tommy35SS
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2008 JK Wrangler 
 
Join Date: Dec 2009
Location: Texas City, TX
Posts: 61
Brake booster operation

I have a 2008 JKU Rubicon 6 speed I bought last March used with 65,000 miles on the clock. I have 75k now. I tried searching but I found nothing under the JK section or any issue that really related to my issue.

The brakes always worked strange from every other vehicle I've driven. When shutting off the engine the brake pedal immediately gets hard and pushes back into my foot shoving it back unless I use a lot of my strength to hold it down. The other time its been different is when panic stopping. The pedal gets hard after about 1/3 of pedal travel and the Jeep doesn't stop fast at all. Its not locking up the tires either. If I press super hard past that point it will stop much faster. It just takes brute strength. Its been this way since I bought it but I just really noticed how bad it was towing/carrying heavy loads and its really disturbing.

I'm thinking maybe a vacuum leak in the booster and/or the check valve is bad? What's y'alls opinions or experience? Is it normal with just crappy factory brakes?

My old TJ could pretty much lock up the tires (no abs) at any speed below 30. My JK's abs system has only kicked in with brute force on the brakes and 5 mph or less

Also when in 4 lo giving throttle then quickly hitting the brakes I have no power brakes at all. Also scary at times when on ledges and what not

Thanks for any help

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Unread 09-18-2013, 02:45 AM   #2
Rtone1583
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2009 JK Wrangler 
 
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"The pedal gets hard after about 1/3 of pedal travel"

Assuming this to mean that it feels like you are braking with no booster assistance.

It sounds like you might have a small vacuum leak to the booster or possibly in the booster. Basically while running at speed there is enough vacuum to get you that first little bit of a panic stop and then the vacuum falls off as the engine speed comes down (this would also explain the brake problem while wheeling in 4LO).

If you've got a vacuum pump it would be easy to check that the system is holding a vacuum and if you don't an easy first step would be to replace the fittings and vacuum hose going from the booster to its vacuum source.
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Unread 09-18-2013, 08:26 PM   #3
ronjenx
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2008 JK Wrangler 
 
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Location: Maine
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Just so you know, manifold vacuum increases when you let off the gas.
Manifold vacuum is higher at idle. (more inHg vacuum)
Manifold vacuum is lower with an open throttle under load. (less inHg vacuum)

Those old enough to remember vacuum powered windshield wipers have seen this first hand.

The 3.6L engine in the 2012+ JKs may be a little different, hence the supplemental electric vacuum pump.

I just went out and tried mine.
Engine running, normal brake pedal.
Keep pedal pushed down, turn off engine, no pedal kick-back.
Can get one more normal vacuum assisted brake application.
Second application, still have some vacuum assist, and pedal gets hard about halfway down.
Third application, the pedal is hard at the top like non-power brakes.
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Unread 09-20-2013, 03:31 PM   #4
JohnRodriguez
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1989 YJ Wrangler 
 
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My 08 had the same problem after I had a LS installed in it. Turn out the O-ring that sealed the mater cylinder to brake booster was not installed when put back togerther. wasn't really a problem for me as I installed my ARB air compressor back on it when I brought the jeep back home, so I just put a new o-ring back on. the o-ring is 1-7/8 ID if yours is missing or broken.

you don't need a vacuum pump to check and see if it is working. just run the engine for about a minute and shut it off. then pull the check valve off. if there is no leak then you will here the air rushing into the booster. It may help to pull it off first before you start and spay the end with some silicone spray, it will help seal also. If you pull the hose off the check valve nothing will happen, took me about two trys before I realised this and then pulled the check valve to booster off.
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Unread 09-21-2013, 03:58 AM   #5
tommy35SS
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I did check the check valve at least. There was no vacuum inside the brake booster when I removed the valve. I'm going to try again today though to see if there is for sure. Thanks for all the suggestions everybody. I'm thinking now there is a small leak in the booster itself, hopefully I'll find out this weekend for sure what it is
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Unread 01-02-2014, 05:50 PM   #6
tommy35SS
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I never finished my thread. Well turns out it was a small leak in the brake booster, it was a real pain in a$$ to change too. If anyone ever has to change their brake booster the holes in the body for the stud from the brake booster to run through are too small to back the nuts out of along with the fact that they are not overlayed correctly. They are off by about 2 millimeters. I had to take a dremel to cut up the body to change the brake booster. I'll post pics when I get a chance
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Unread 01-02-2014, 08:44 PM   #7
JohnRodriguez
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check the o-ring that is between the master cylinder and the booster. if it is cracked or missing then it will cause your problem. I believe it is a 7/8 id and 1" od with a 1/16 thick.


Just read your last post. the o ring was the problem that I found on my 08
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89 Wrangler, 2.5L TBI, D30 Truetrac 4.56 gear front, Super 35 4.56 gears rear, 31" BFG Mud's, 15x9" weld racing pro-stars XP's, 2.5" rancho and 1.25" shackels, superior axles. rear wilwood disc brakes

2008 wrangler x unlimited. Stone white. Motech 5.3L LS Chevy
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