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#4697187 - 03/16/18 10:57 PM What is a spongy pedal?
motor_oil_madman Offline


Registered: 11/29/09
Posts: 4717
Loc: Houston, Texas
What do people mean by a spongy brake pedal? You know how you can press the brakes hard and the pedal is firm, but then you press it harder and it goes down say another 1/4" but the vehicle doesn't really slow down any faster. Is that a spongy pedal?
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#4697188 - 03/16/18 10:58 PM Re: What is a spongy pedal? [Re: motor_oil_madman]
bdcardinal Online   content


Registered: 06/03/05
Posts: 10808
Loc: Santa Barbara, CA
Go find a wet sponge and step on it. That is what it feels like.
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#4697273 - 03/17/18 05:27 AM Re: What is a spongy pedal? [Re: motor_oil_madman]
d00df00d Offline


Registered: 10/20/05
Posts: 11126
Loc: PA
Ideally, when you press the brake pedal, it should feel hard as a rock. There can be a tiny bit of dead space at the top of the pedal travel, but as soon as the brakes start grabbing, the pedal shouldn't give. Pushing harder should make you stop faster, but it shouldn't make the pedal move. That way, you can control your braking with pedal pressure, not pedal travel. It's more intuitive and confidence-inspiring, and gives the best feedback.

Cars are rarely engineered that way, unfortunately. There's always a little bit of "give" in the pedal. Some vehicles have more than others. But the pedal should still be firm when the brakes are engaged, and you should still be controlling your brakes mainly with pedal pressure, not pedal travel.

If it's not like that, it's spongy.

And if pressing harder makes the pedal move but doesn't make the vehicle decelerate any faster, that's even worse. It's the opposite of how the pedal should work.
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#4697308 - 03/17/18 07:24 AM Re: What is a spongy pedal? [Re: motor_oil_madman]
motor_oil_madman Offline


Registered: 11/29/09
Posts: 4717
Loc: Houston, Texas
Ive noticed some vehicles take very little effort to push the brake pedal. The brakes work very well, the pedal is just very touchy.
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#4697457 - 03/17/18 10:22 AM Re: What is a spongy pedal? [Re: motor_oil_madman]
mk378 Offline


Registered: 09/27/15
Posts: 1327
Loc: USA
On any car:
* Pressing on the brake pedal with the engine running but car not moving, the pedal should not go all the way to the floor.
* With the car moving, the brakes should lock up well before the point you reached in the first test.


Edited by mk378 (03/17/18 10:23 AM)

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#4697992 - 03/17/18 08:31 PM Re: What is a spongy pedal? [Re: motor_oil_madman]
vavavroom Offline


Registered: 10/31/17
Posts: 87
Loc: California
Dear yonder varlet,

Air or water, unlike uncontaminated brake fluid, in the brake fluid is compressible. Hence the pedal giving spongy and indirect versus firm and linear feedback.

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#4698083 - 03/17/18 09:47 PM Re: What is a spongy pedal? [Re: motor_oil_madman]
AVB Offline


Registered: 05/20/12
Posts: 1140
Loc: Georgia
I don't think water is compressible. It will boil though.

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#4698267 - 03/18/18 04:42 AM Re: What is a spongy pedal? [Re: motor_oil_madman]
Kibitoshin Offline


Registered: 04/19/16
Posts: 656
Loc: Kaiōshinkai
Typical vacuum booster behavior.
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#4698358 - 03/18/18 08:05 AM Re: What is a spongy pedal? [Re: AVB]
d00df00d Offline


Registered: 10/20/05
Posts: 11126
Loc: PA
Originally Posted By: AVB
I don't think water is compressible. It will boil though.

Everything is compressible. The question is how much. Water has minimal compressibility, but not zero. Same with brake fluids.

Not sure if liquid water is more or less compressible than brake fluid at the relevant temps, though.
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2011 Mazda RX-8 R3
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#4698592 - 03/18/18 11:59 AM Re: What is a spongy pedal? [Re: Kibitoshin]
AVB Offline


Registered: 05/20/12
Posts: 1140
Loc: Georgia
Originally Posted By: Kibitoshin
Typical vacuum booster behavior.

I agree, it could be flexing, but I thought he may be talking about his diesel. It should have a hydroboost.

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#4699845 - 03/19/18 03:18 PM Re: What is a spongy pedal? [Re: AVB]
vavavroom Offline


Registered: 10/31/17
Posts: 87
Loc: California
Originally Posted By: AVB
I don't think water is compressible. It will boil though.

You are correct, of course. The water in the brake fluid becomes compressible only once it vaporizes, which will happen under hard braking. Liquid water is measurably compressible, but it takes a whole lot of compression.

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