Rear Brake Pads wore down to METAL in 24k miles! - Page 2 - Mercedes-Benz Forum
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post #11 of 25 (permalink) Old 08-12-2015, 08:54 AM Thread Starter
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I should have clarified better...I do on one side but the other would too in a 1000 miles because the pad was down to 1mm left of actual pad.

Brake hoses are new I posted earlier.
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post #12 of 25 (permalink) Old 08-12-2015, 09:07 AM
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Originally Posted by ps2cho View Post
BOTH sides are worn to the metal. This isn't just one side.

I've read the odds of two bad calipers at the exact same time are slim to none.
The symptoms indicates that the problem is not with the calipers; it points to something related to fluid flow restriction between the caliper and the brake master cylinder.

Note: This is the same exact experience I had. In my case there was a faulty check valve between the master cylinder and the rear brake calipers. The purpose of this check valve is to regulate the rear brake pressure so that during sudden brake application the rear of the car won't slide or swing to the front. I'm not sure if MB's use something like this.

Check your brake lines for restrictions between the Master cylinder to the rear calipers.

Last edited by edcarls; 08-12-2015 at 09:15 AM.
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post #13 of 25 (permalink) Old 08-12-2015, 09:18 AM Thread Starter
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I did some searching online and it kept coming up with proportioning valve, but I can't find anything like that for MB. Is it integrated in the master cylinder for us maybe?

Just ordered a new set of Zimmerman rotors and Pagid pads so I can at least get it on the road again.
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post #14 of 25 (permalink) Old 08-12-2015, 09:28 AM
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Originally Posted by ps2cho View Post
I should have clarified better...I do on one side but the other would too in a 1000 miles because the pad was down to 1mm left of actual pad.

Brake hoses are new I posted earlier.
Right, I read too quickly, as usual.

Well, if the new set wears quickly, I'd consider trying to flush that one caliper. Perhaps there is some stuff inside from the old hoses?
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post #15 of 25 (permalink) Old 08-12-2015, 10:04 AM Thread Starter
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I'm about close to the 2 year mark at this point anyway so when I get the new pads I'll bleed everything for good measure.
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post #16 of 25 (permalink) Old 08-12-2015, 10:09 AM
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Get the back end of the vehicle in the air.
Have an assistant apply the brakes as you observe how the rear brake calipers react by turning the rear wheels by hand, pedal on, pedal off. I'll bet the wheel locks immediately when the brake is applied, AND the wheel still doesn't turn when the brake pedal is released. After say two seconds, the wheel slowly begins to turn.
I say the calipers are not releasing in a timely manor.
This would indicate a fluid restriction, given the pressure of the brake pedal can easily overcome a restriction, whereas a restriction is NOT easily overcome when the brake pedal is released.
Inspect the entire metal brake line, front to rear for kinks and pinched line, especially the most rearward lines. Did someone drive the vehicle over a curb or drive off road in the last 24K miles?
If you see nothing amiss on the lines, we now focus on the calipers. Remove the wheels, repeat the above test.
Observe the movement of the pads. Look for torn or missing rubber boots on the caliper pistons, and rust buildup on the pad assemblies where the pins attach to the pads.
Remove the pads and carefully apply the brakes just enough to expose a portion of the pistons to look for rust on the piston sides.
Lastly, pads in place, wheels off, apply the brakes. Instead of releasing the brake pedal, crack a brake hose at the caliper(or open a bleeder valve), and see if the immediate release of brake fluid at that point immediately releases the pistons.

I'm confident you have a restriction in a brake line, not bad calipers, and I'm betting it's the hoses, despite the fact you changed them @140 K. Interior rust on those hoses WILL clog and restrict brake fluid.
As Jayare pointed out, the metal lines won't rust shut, they would rust thru causing a leak, but I assure you the hoses DO rust and cause these issues. Can they rust shut in only a short time frame? Sure, if they are not MB original or high quality(stainless) products.
One of the problems with brake fluid is that it is hydroscopic, meaning it attracts moisture, and that can and will rust the interior of brake lines.

Another thing to consider. If there is air in the caliper, it can heat up under heavy braking(or pads dragging on the rotors), causing the caliper to apply the brakes slightly, even when the pedal isn't applied. I doubt this is the case, it takes a lot of heat to produce these symptoms, and you'd probably smell the burning, but it's a possibility.

A malfunctioning check valve, or proportioning valve would give these symptoms as well. I personally have not seen this occur, but I would consider that a possibility.


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post #17 of 25 (permalink) Old 08-12-2015, 10:49 AM
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...with proportioning valve, but I can't find anything like that for MB. Is it integrated in the master cylinder for us maybe?..
My thoughts exactly. Since, front and rear brake lines are coming out separately from the master cylinder it make sense that the "proportioning" function is done at the master cylinder. Thus, MB eliminated the valve.

If the line from the brake master cylinder to the rear calipers are clear, and the problem still persist, a brake master cylinder repair kit my solve your sticky rear brake problem.

Last edited by edcarls; 08-12-2015 at 02:36 PM. Reason: to add, "and the problem still persist," for clarity.
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post #18 of 25 (permalink) Old 08-12-2015, 11:02 AM Thread Starter
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Great info above thanks. I'll check that after I install the new rotors and pads Saturday and report back.

Last edited by ps2cho; 08-12-2015 at 11:06 AM.
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post #19 of 25 (permalink) Old 08-12-2015, 11:19 AM
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Quote:
Originally Posted by ps2cho View Post
Ok so to clarify...you think my best course of action would be to bleed the brakes and if nothing odd shows up replace the master cylinder?

Or is there a better way to check for restrictions?
Do the bleed, don't replace the MC at this point.

While bleeding pay attention to how much fluid comes out of each caliper. If you notice very little from the rears compared to a lot from the fronts, bingo, there is your restriction.
Then pull those hoses, and see how much fluid comes out of the metal lines. I'm not giving up on hoses till you PROVE they are clear.

With all the hoses removed, fluid should gush out of those brake lines when you hit the pedal. Have your assistant keep the fluid topped off.

If your dual piston master cylinder is faulty, it COULD be sending the majority of the braking force to the rear pads. What kind of wear did you notice on the front pads over the same time period? I would think you would see substantial wear on the fronts after 24K miles.
How does the pedal feel? Does the back end want to come around under braking on slippery surfaces?

Have you researched MC failures on the forum?
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post #20 of 25 (permalink) Old 08-12-2015, 05:33 PM
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Remember that the master cylinder has two separate pistons in it not one. So when you push the pedal down, both pistons move forward. They should both return on their own but one can stick causing the pads related to the sticking piston to apply friction to the rotors when it shouldn't. When this happens it's usually the forward piston - the one that supplies the rears with fluid. Its rare that hoses will cause your problem because they would both have to suffer the same fate - highly unlikely they would both fail after you just changed them but I guess anything is possible. A master cylinder problem would affect both rears at the same time. Just food for thought.

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