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Old November 17th, 2019, 19:46
mhead mhead is offline
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Location: Encinitas, CA
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Brake Booster Weak?

Hi All,

I've a 1992 XJ 4.0 4x4 auto tranny. I have about 3K miles on new pads and rotors. I've tried two types of new pads. This vehicle HAD the ABS unit, which I removed. I simply routed master cylinder outs to front and to back brake hydraulic lines, eliminating the ABS.

Under normal brake pressure this jeep stops normally. However in an emergency situation very hard brake pressure does not cause the front wheels to lock up. It seems like the booster assists up to some point but after this point further braking force must be applied by foot onto pedal without much boost. i don't recall when this problem appeared.

When started with foot on the pedal the pedal sinks in the expected way.

After stopping engine the brake assist seems only to last about 30 seconds. I guess the one-way vac valve has a leak or possibly there is some other leak into the booster.

I'm casting about for some explanation. I suppose if the manifold vacuum were low then there would be a limit to the amount of boost I could expect. Anyone know the expected vacuum at idle or some RPM?

Maybe there's a leak in the booster that lowers the overall vac available?

I'm really lost on this one and will appreciate suggestions on how to test the booster or other parts in the system.

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Old November 18th, 2019, 09:30
lazyxj's Avatar
lazyxj lazyxj is offline
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Join Date: Mar 2004
Location: Los Angeles
Posts: 2,619
Re: Brake Booster Weak?

Originally Posted by mhead View Post
Hi All,
After stopping engine the brake assist seems only to last about 30 seconds. I guess the one-way vac valve has a leak or possibly there is some other leak into the booster.
If the vacuum disappears in 30 second, you have a serious leak, the booster should hold vacuum for a long time.
Pull the check valve and see if it is airtight in one direction and make sure it seals highly in the booster, I like to coat the check valve with a little dielectric grease for a better seal.
If it still losses vacuum, it's time for a new booster. Good chance the booster is 27 years old and the diaphragm has deteriorated .
Both XJs are still Real XJs
01 Sport, Russ Pottenger 4.6, AW4, 4" lift, 33 X 10.50 KM2, 4.56-8.25 TruTrac, D-30-E-Locker with alloy shafts, Hidden M8000 winch, Vanco front and ZJ rear disc brakes, Frame stiffeners, Lots of armor, Towing baseplate.
1997 Country, Mostly stock, 2" lift, Engine mods, NP242, Towing baseplate
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Old November 19th, 2019, 14:17
denverd1 denverd1 is offline
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Join Date: Dec 2013
Location: Tyler, TX
Posts: 840
Re: Brake Booster Weak?

yes. "hard braking" isn't doing it any favors either. blew mine on a serious downhill section of trail.

you can hear from the driver's seat when the booster is bad.
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