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What causes the first application of the brake pedal to have considerable (alarming) distance before the brakes engage, but then quickly letting off and re-applying, the pedal stroke is considerably shorter and the feel is nice and firm??
Air in lines?
Vacuum assist / power brake booster faulty?
Messed up master cylinder?
Some combination of above?
How to troubleshoot?
Thanks.
 

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Could be air. What is the history, what's been done to the car? Was it OK at some time before this? What has changed?
 

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You can rule out the booster. The booster would make the pedal hard if it was faulty.
I would vote for some combination of air in the lines or the master cylinder. Or a restriction like deteriorated flex hoses or corroded caliper slides.
 

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Adjust your drum brakes.
Drum brakes will give you a long pedal if they are never adjusted or if the remaining lining suddenly disintegrates.
 

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That won't cause the pedal to behave that way.
 

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I'm running into the same problem with my car now. The problem is intermittent. When the car is cold the pedal has to go down considerably before I begin stopping when I apply the brakes the first couple of times. As the car warms the pedals act normally unless I have to brake hard. If I brake hard, then the pedal will have normal travel for a few seconds and then "give up" and travel all the way down. The car will still stop though. Once I pump the brakes in this situation, the braking returns to normal. Brake booster is relatively new (circa 2006), and it does not make a hissing noise or anything that would suggest to me that its leaking. The master cylinder however is original (1984). I initially thought it was a problem with the vacuum, but my cousin is positive its the master cylinder. Anyone have thoughts?
 

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I'm running into the same problem with my car now. The problem is intermittent. When the car is cold the pedal has to go down considerably before I begin stopping when I apply the brakes the first couple of times. As the car warms the pedals act normally unless I have to brake hard. If I brake hard, then the pedal will have normal travel for a few seconds and then "give up" and travel all the way down. The car will still stop though. Once I pump the brakes in this situation, the braking returns to normal. Brake booster is relatively new (circa 2006), and it does not make a hissing noise or anything that would suggest to me that its leaking. The master cylinder however is original (1984). I initially thought it was a problem with the vacuum, but my cousin is positive its the master cylinder. Anyone have thoughts?
You are describing a hydraulic problem. It's not the booster. All the booster does is make it easier to press the pedal. The pedal dropping is almost always a master cylinder. To know what a failed booster is like, press the brake pedal with the engine off. There is still good braking, you just have to push really hard.
 

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If brake parts or wheel bearings are mechanically worn or loose, the pads may be moved away from the rotor on the first brake application. the pedal will be low. The subsequent pedal applications will-pump-up the pedal for a good stop.
If the pedal 'sinks' while applied after a moment and there are no external fluid leaks, then I would suspect that the m/cylinder has an internal leak.
 

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You are describing a hydraulic problem. It's not the booster. All the booster does is make it easier to press the pedal. The pedal dropping is almost always a master cylinder. To know what a failed booster is like, press the brake pedal with the engine off. There is still good braking, you just have to push really hard.
Yea I'm 100% its not the booster. I'm holding off on driving until i can take it to my cousins to get the master cylinder replaced. The lines are fused pretty good since they've been connected for so long.
 

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If brake parts or wheel bearings are mechanically worn or loose, the pads may be moved away from the rotor on the first brake application. the pedal will be low. The subsequent pedal applications will-pump-up the pedal for a good stop.
If the pedal 'sinks' while applied after a moment and there are no external fluid leaks, then I would suspect that the m/cylinder has an internal leak.
Yea i suspect it too. The car is leak free all the way around and I'm not losing fluid in the container.
 

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That won't cause the pedal to behave that way.
Yeah, it will. The shoes are too far from the drums after sitting and retract slowly so they are quicker when immediately reapplied.
 
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If brake parts or wheel bearings are mechanically worn or loose, the pads may be moved away from the rotor on the first brake application. the pedal will be low. The subsequent pedal applications will-pump-up the pedal for a good stop.
If the pedal 'sinks' while applied after a moment and there are no external fluid leaks, then I would suspect that the m/cylinder has an internal leak.
I agree, air in the system will act the same way.
 

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Finally got everything replaced, new master cylinder, booster, shoes, pads, lines to the proportioning valve, drums, springs in the drums, and wheel cylinders along with a bled brake system. The result its stops a lot better but still feels spongy. You really have to hold your foot up to keep from slamming the pedal to the floor. Like there's no push back from the pedal. If you do push the pedal to the floor, you're going to stop. HARD. I've not push the pedal more than halfway under normal operation. Its just soft.
 

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If this is a minivan then I would start by making sure the rear drums are adjusted. Minivans have a crossed or that X style brake system where the right front brake is connected to the left rear drum brake.

If the rear drums are out of adjustment this can give you a longer pedal stroke. If while pressing on and holding the brake pedal and the pedal slowly falls away from your foot. Then you have a leak.
 

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That's what's happening. The shop said they adjusted the rear brakes so I feel like I can rule that out. The brakes are consistently firm at idle or under heavy acceleration. As the car decelerates though it has this problem. This was the same problem I had before I did all this work on the car so I'm finding it hard to believe it is yet another master cylinder. I initially thought vacuum, but that would give me the opposite problem wouldn't it?
 

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Is the "new" master cylinder really new or a reman one? I've heard horror stories on the reman ones being bad right out of the box.
 

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Is the "new" master cylinder really new or a reman one? I've heard horror stories on the reman ones being bad right out of the box.
You're right. The first new I had was bad out of the box (reman Chrysler). This is the second new one (Actually new). Its still exhibiting the same problems.
 

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Probably a stupid question, but have you checked the check valve on the power brake booster? It could be faulty and not pulling a vacuum like it's supposed to.
 

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Probably a stupid question, but have you checked the check valve on the power brake booster? It could be faulty and not pulling a vacuum like it's supposed to.
The brake booster has been replaced once already. I didn't recall seeing a check valve anywhere on it. Where would that be?
 
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