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What is a spongy pedal? #4697187
03/16/18 10:57 PM
03/16/18 10:57 PM
Joined: Nov 2009
Posts: 4,977
Houston, Texas
motor_oil_madman Offline OP
motor_oil_madman  Offline OP
Joined: Nov 2009
Posts: 4,977
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?


2007.5 dodge cummins 6.7 liter. Chevron Delo400 15w40. 7000 mile or 250-300hr intervals.

Re: What is a spongy pedal? [Re: motor_oil_madman] #4697188
03/16/18 10:58 PM
03/16/18 10:58 PM
Joined: Jun 2005
Posts: 11,409
Santa Barbara, CA
bdcardinal Online content
bdcardinal  Online Content
Joined: Jun 2005
Posts: 11,409
Santa Barbara, CA
Go find a wet sponge and step on it. That is what it feels like.


2014 Ford Mustang GT Track Pack
1995 Ford Mustang GT

Ford/Mazda Parts Counter
NRA Benefactor Member
Opinions expressed are my own.
Re: What is a spongy pedal? [Re: motor_oil_madman] #4697273
03/17/18 05:27 AM
03/17/18 05:27 AM
Joined: Oct 2005
Posts: 11,383
PA
d00df00d Offline
d00df00d  Offline
Joined: Oct 2005
Posts: 11,383
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.


2011 Mazda RX-8 R3
Mobil Super 5w-20
Re: What is a spongy pedal? [Re: motor_oil_madman] #4697308
03/17/18 07:24 AM
03/17/18 07:24 AM
Joined: Nov 2009
Posts: 4,977
Houston, Texas
motor_oil_madman Offline OP
motor_oil_madman  Offline OP
Joined: Nov 2009
Posts: 4,977
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.


2007.5 dodge cummins 6.7 liter. Chevron Delo400 15w40. 7000 mile or 250-300hr intervals.

Re: What is a spongy pedal? [Re: motor_oil_madman] #4697457
03/17/18 10:22 AM
03/17/18 10:22 AM
Joined: Sep 2015
Posts: 1,552
USA
mk378 Offline
mk378  Offline
Joined: Sep 2015
Posts: 1,552
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.

Last edited by mk378; 03/17/18 10:23 AM.
Re: What is a spongy pedal? [Re: motor_oil_madman] #4697992
03/17/18 08:31 PM
03/17/18 08:31 PM
Joined: Oct 2017
Posts: 260
California
vavavroom Offline
vavavroom  Offline
Joined: Oct 2017
Posts: 260
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.

Re: What is a spongy pedal? [Re: motor_oil_madman] #4698083
03/17/18 09:47 PM
03/17/18 09:47 PM
Joined: May 2012
Posts: 1,401
Georgia
AVB Offline
AVB  Offline
Joined: May 2012
Posts: 1,401
Georgia
I don't think water is compressible. It will boil though.

Re: What is a spongy pedal? [Re: motor_oil_madman] #4698267
03/18/18 04:42 AM
03/18/18 04:42 AM
Joined: Apr 2016
Posts: 763
Specific Ocean
Kibitoshin Offline
Kibitoshin  Offline
Joined: Apr 2016
Posts: 763
Specific Ocean
Typical vacuum booster behavior.


Shin Gekiretsu Shin'ou'hou
01 Toyota Tundra 2WD V8 4.7L - 170k mi
02 Chevy Silverado C1500 V8 5.3L 154k mi
Re: What is a spongy pedal? [Re: AVB] #4698358
03/18/18 08:05 AM
03/18/18 08:05 AM
Joined: Oct 2005
Posts: 11,383
PA
d00df00d Offline
d00df00d  Offline
Joined: Oct 2005
Posts: 11,383
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.


2011 Mazda RX-8 R3
Mobil Super 5w-20
Re: What is a spongy pedal? [Re: Kibitoshin] #4698592
03/18/18 11:59 AM
03/18/18 11:59 AM
Joined: May 2012
Posts: 1,401
Georgia
AVB Offline
AVB  Offline
Joined: May 2012
Posts: 1,401
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.

Re: What is a spongy pedal? [Re: AVB] #4699845
03/19/18 03:18 PM
03/19/18 03:18 PM
Joined: Oct 2017
Posts: 260
California
vavavroom Offline
vavavroom  Offline
Joined: Oct 2017
Posts: 260
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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