Spongy Brake Pedal

[Scott Feit]

I drove my friend’s 2001 Mercedes ML the other day and was surprised when I found I needed to press the brake pedal about halfway to the floor before the brakes responded. After that they seemed to work normally. I suspect there is just a lot of air in the lines, but my question is is this the only cause of a spongy pedal? Is there anything else that could cause the brakes to act this way? I ask because I don’t want to bleed her brakes and end up going “oh… they’re still broken… sorry I just wasted an hour of your time… might wanna get that looked at.” Because honestly, beyond bleeding the brakes I don’t know what else to try.

Topic

[college man]

Has there been any brake work done recently? how much pad
life is left? brake fluid full? condition of the fluid?
drum rear brakes or 4 wheel disc?

[Scott Feit]

[quote=”college man” post=56378]Has there been any brake work done recently? how much pad
life is left? brake fluid full? condition of the fluid?
drum rear brakes or 4 wheel disc?[/quote]

I don’t know… and somehow I doubt she does either. Would normal pad wear actually cause a spongy brake pedal though? Logically I would think the only effect would be for the brake pedal to get closer to the floor; I don’t understand how this could allow air to enter the system, but then again I’m a noob :unsure:

I’ll ask her about it when she gets home later today and hopefully I’ll get a better look at the car sometime this week.

[college man]

If the pads are on there way out. The piston will not
get the correct fluid pressure due to the over extension
of the piston.

[Tanner]

I had this happen to me three times, the first time was because my brake calipers were leaking, the second time was because I had the back brake shoes replaced and they didn’t adjust them after they put em in. And last, was when the master cylinder went out. Just something’s to look into.

The brake system is a sealed system so you should never lose fluid, as said befor when the pads get used up it takes more brake fluid to push the pistons but the fluid shouldn’t be leaving the system

[Scott Feit]

[quote=”college man” post=56385]If the pads are on there way out. The piston will not
get the correct fluid pressure due to the over extension
of the piston.[/quote]

Today I was able to do a quick visual inspection, but I have yet to jack it up and start popping wheels off.

-4 wheel disks
-pad wear does not seem excessive… at least on the side I could see
-brake fluid is topped off to the max
-fluid type is DOT 4+, because Mercedes has to be different (and I think the only place I can get some locally is the stealership for $15/liter or so)
-color is sort of dark brown, but not too dark… definitely still translucent
-she says it’s been this way for about 6 months, no work done to it
-braking full force had no effect (ABS seemed to kick in)

Once I get my tools back from my friend (hopefully this week) we’ll bled the brakes. Hope that’ll solve it….

Thanks for your help.

[Tanner]

Hmmm maybe trace the brake lines too, I know a guy that had a similar problem and one of the brake lines just flat out broke. But it should be pretty obvious if one did. Now watch your fluid if it get low it’s getting out somewhere. Also, is the abs light on? And it sounds kind of like the master cylinder let us know what you find out

Good luck

[Scott Feit]

[quote=”Passey” post=56548]Hmmm maybe trace the brake lines too, I know a guy that had a similar problem and one of the brake lines just flat out broke. But it should be pretty obvious if one did. Now watch your fluid if it get low it’s getting out somewhere. Also, is the abs light on? And it sounds kind of like the master cylinder let us know what you find out

Good luck[/quote]

Thanks for the advice. Like I said the master seemed full, and she said it’s been this way for 6 months so I don’t think there’s a leak. I looked for traces of brake fluid around the wheels (hard to see the lines with the truck still on the ground) and didn’t see anything suspicious. No lights on or anything, and ABS seemed to do its job when I tried slamming the brakes earlier.

[Tanner]

Does it pull to one side or pulsate when your braking?

[Scott Feit]

[quote=”Passey” post=56554]Does it pull to one side or pulsate when your braking?[/quote]

Not at all. Seems perfectly normal, only symptom is there is no brakes for the first half of pedal travel.

[Tanner]

Well it sounds like a master cylinder to me then, when mine went out sometimes it would work and sometimes not if I pumped it while braking it worked better, I don’t know how he car works I have never been around them but that’s what I would look into.

[Trent “Teddy” Martin]

The first thing to do is bleed the system, its easy doesn’t cost any and it can fix a lot. If the didn’t work the next thing I’d do is change fluid. Moisture within brake fluid can cause you to have no pedal every so often then a hard pedal the rest of the trip. If that doesn’t work reply and I got some more tips for you 🙂

[Author]

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