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  #1  
Old September 1st, 2015, 07:31
md21722 md21722 is offline
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Brake pedal drops when I start the Jeep

On my 97 XJ the brake pedal drops a few inches when I start the Jeep. If I pump it up it stays high. If I shut it off an immediately restart, it will drop a few inches again. And again. My 01 XJ doesn't do this. What are the likely causes or how can I troubleshoot?
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  #2  
Old September 1st, 2015, 08:01
mtbxj87 mtbxj87 is offline
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Re: Brake pedal drops when I start the Jeep

Probably the brake booster. Make sure your brakes are bled and shoes adjusted, no leaks. The pedal should be firm without the jeep running. You can try capping off the vacuum line to booster and see if the pedal drops when you start it.
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  #3  
Old September 1st, 2015, 09:04
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sidewaysstarion sidewaysstarion is offline
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Re: Brake pedal drops when I start the Jeep

That's completely normal, when you pump it you use up all of the vacuum in the brake booster and when you start the engine the engine supplies the booster with more vacuum with allows the pedal to travel down.

I can't say I've ever driven a car that what you describe DIDN'T happen.
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Old September 1st, 2015, 10:06
md21722 md21722 is offline
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Re: Brake pedal drops when I start the Jeep

Put differently, every time I start the engine I have to pump the brake pedal several times so it doesn't have a lot of travel. I can drive for miles and the extra travel does not come back until I restart the engine. I will try capping the booster and see how that works.
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Old September 1st, 2015, 11:50
cnugent cnugent is offline
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Re: Brake pedal drops when I start the Jeep

Yeah, I think what you're getting is the power assist kicking in from the booster when the engine is started. Try starting it without having your foot on the pedal already. Once started, then give it a good press and see what happens.

If you can pump it a couple times with the engine off and you have a nice hard pedal, I don't think you have a problem.
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Old September 1st, 2015, 12:47
md21722 md21722 is offline
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Re: Brake pedal drops when I start the Jeep

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Originally Posted by cnugent View Post
Yeah, I think what you're getting is the power assist kicking in from the booster when the engine is started. Try starting it without having your foot on the pedal already. Once started, then give it a good press and see what happens.

If you can pump it a couple times with the engine off and you have a nice hard pedal, I don't think you have a problem.
If I start the engine without having my foot on the pedal, then the same travel comes back. Also with the engine running, and pumping the brake pedal, the travel will not go away unless I do some driving. If I shut the engine off, I can pump the pedal a few times and take out all the pedal travel. My 01 XJ does not do this, the pedal travel never increases or decreases engine on or off, never need to pump.
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Old September 1st, 2015, 13:29
trippled trippled is offline
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Re: Brake pedal drops when I start the Jeep

The booster holds reserve vacuum so if the engine dies you will have a few assisted brake applications. Thats why if you pump it engine off it gets hard. Do a complete check of your hydraulics, as I have seen master cylinders do this, before condemning the booster
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Old September 1st, 2015, 14:31
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techno1154 techno1154 is offline
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Re: Brake pedal drops when I start the Jeep

The pedal dropping when the engine is started is normal in every vacuum assist break vehicle. If the drop is excessive and a few strokes of the pedal causes it to get much higher then there may be something wrong. Most likely not serious but need some attention.

You need to make certain there is no air in the break system. Also inspect the lines for leaking break fluid. The rear drum should be removed and everything behind there inspected. Lastly, adjust the rear breaks. The shoe may be too far from the drum requiring a second and third stroke of the pedal.
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Old September 1st, 2015, 19:47
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Johnnie Walker Johnnie Walker is offline
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Re: Brake pedal drops when I start the Jeep

Make sure that the valve at the booster is working properly as well.
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  #10  
Old September 3rd, 2015, 09:03
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Re: Brake pedal drops when I start the Jeep

I'd be more worried about the '01's lack of change.
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  #11  
Old September 5th, 2015, 14:26
md21722 md21722 is offline
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Re: Brake pedal drops when I start the Jeep

I solved the problem.

I had previously bled the brakes and it didn't seem to help. I tried the method of "take out the brake light fuse, and hold the brake pedal down overnight". I did this twice. I didn't solve the problem. Then I bled the brakes again.

Now the pedal dips slightly when I start the engine but that's it. Otherwise it seems normal.

Thanks!
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  #12  
Old April 8th, 2016, 00:37
md21722 md21722 is offline
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Re: Brake pedal drops when I start the Jeep

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Originally Posted by md21722 View Post
I solved the problem.

I had previously bled the brakes and it didn't seem to help. I tried the method of "take out the brake light fuse, and hold the brake pedal down overnight". I did this twice. I didn't solve the problem. Then I bled the brakes again.

Now the pedal dips slightly when I start the engine but that's it. Otherwise it seems normal.

Thanks!
The above didn't permanently solve the problem. I've since changed the wheel cylinders and now its pretty much the same as the 01 XJ... I think I had a leaky rear wheel cylinder that was allowing air to enter the system. Since replacing them, the brakes are better than they ever have been.
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