It finally got cold in Syracuse, NY. I had to brake rather hard yesterday to avoid a deer and I heard a sort if high pitched moan from under the dash. The brakes seemed fine after that unless I braked hard, then it happened again. This is a 00 WJ with 135K on it.

My question is, is this a vacuum leak in the booster or something else? I did notice the pedal went lower when this happens, but recovers after. This noise is not the ABS. I know what that sounds like. The engine seems to run and idle fine like there is no leak. Should I be looking to replace the booster?

Opinions, please.

 

Sounds like a booster. Mine does the same thing. Just have not had the time to fix it yet.

 

Quickest way to test brake booster vacuum is to pump up the brakes until it is hard and doesnt drop any more while the engine is off. Then while holding the brake, start the engine and hope it drops. Also once its running and your holding the brake, rev the engine and you should feel it drop slightly more. Also you can try spraying a flammable liquid on the vacuum lines and see if the rpms jump. Brake clean or carb clean should work. Dont rev the engine during this step since the more open the throttle plate is the less vacuum there is.

 

Okay, I'll try the tests. I'm thinking the leak is under the dash because that's where the noise is coming from, probably the pedal shaft seal. With the engine off, the pedal is rock solid so I know the hydraulics are good. This problem is only on a quick hard brake application and not with normal usage. Last night we got 2 feet of heavy wet snow and while delivering newspapers, I acually got into ABS function several times. No noise then. The pedal pulsed as it should and the brakes chattered as they should, but no noise. Weird, huh? The engine doesn't stagger or miss like you would expect with a major vacuum leak and the brakes will stop the car fine, so I think this is a fair weather repair. I just get queasy with ANY problems with brakes, haveing lost the brakes on a fully loaded pickup going downhill!!

Life's experiences make you hope it never happens again!