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  #16  
Old 08-24-2006, 11:13 AM
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Quote:
Originally Posted by rrgrassi View Post
All it takes is the brake pedal to drop out from under your foot while trying to stop. .
This must have happened on an old single chamber master cylinder??? What are the odds of a front and back system failure at the same time?

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  #17  
Old 08-24-2006, 11:35 AM
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Actually it was on a dual system. The right rear wheel cylinder seal popped. It was on an older Chevy P/U. Front disc, rear drum. Icy conditions along I-20. That was not a good time to find out about the problem. Ever since then I check the brake system every time I rotate tires.
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70's Southern Pacific #5608 Fairmont A-4 MOW car

13 VW JSW 2.0 TDI 193K, Tuned with DPF and EGR Delete.

91 W124 300D Turbo replaced, Pressure W/G actuator installed. 210K

90 Dodge D250 5.9 Cummins/5 speed. 400K
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  #18  
Old 08-24-2006, 11:41 AM
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Quote:
Originally Posted by rrgrassi View Post
Actually it was on a dual system. The right rear wheel cylinder seal popped. It was on an older Chevy P/U. Front disc, rear drum. Icy conditions along I-20. That was not a good time to find out about the problem. Ever since then I check the brake system every time I rotate tires.
On a dual system, the brake pedal can't go to the floor when one system fails.
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  #19  
Old 08-24-2006, 12:04 PM
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Quote:
Originally Posted by Brian Carlton View Post
On a dual system, the brake pedal can't go to the floor when one system fails.
Actually it can. You will feel some resistance just before the pedal touches the carpet. At that point, your proportioning valve should have given full power to the remaining system.
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70's Southern Pacific #5608 Fairmont A-4 MOW car

13 VW JSW 2.0 TDI 193K, Tuned with DPF and EGR Delete.

91 W124 300D Turbo replaced, Pressure W/G actuator installed. 210K

90 Dodge D250 5.9 Cummins/5 speed. 400K
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  #20  
Old 08-24-2006, 12:27 PM
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Originally Posted by rrgrassi View Post
Actually it can. You will feel some resistance just before the pedal touches the carpet. At that point, your proportioning valve should have given full power to the remaining system.
Such a scenario would defeat the entire purpose of a dual m/c with separate braking capability on each system. There must be more to the story.
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  #21  
Old 08-24-2006, 01:31 PM
Craig
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Originally Posted by rrgrassi View Post
Actually it can. You will feel some resistance just before the pedal touches the carpet. At that point, your proportioning valve should have given full power to the remaining system.
I drove mine about 500 miles with a broken rear line and the pedal was pretty low, but not to the floor. The other brakes still worked.
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  #22  
Old 08-24-2006, 02:05 PM
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Look at flexible lines and brake fluid

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Originally Posted by rblookc View Post
Looking for input on this issue ...

I changed out all the pads in my 85 300D this weekend. Three out of four wheels looked normal. Pads were worn. Rotor ok not great. BUT, the left front unit was toast. The pad material was crumbly. The pad housing was nearly impossible to remove from the caliper. I completed the job. The performance is WAY better but there is still a noticeable brake shudder at high speeds. Now I am wondering ...

1) If I now decide to change the front rotors, how soon need I do it before the new pads are too old to put on new rotors?
2) Is it possible a faulty LF caliper caused the pad problem? It seemed heat damaged and the pad housing was corroded.
3) I did notice that the rubber boot on the caliper piston on the RIGHT front caliper was disintegrated (the boot you see when the pad is removed and the piston is all the way out). Is this a sign that my calipers need an overhaul? Or can I work it out. There is no pull or other blatant brake problem.

Thanks in advance.
Flexible brake lines can clog so that when you put pressure on the pedal you get brakes, but when you take your foot of off the brake pedal the caliper "hangs up" and the pistons in the caliper don't go back in their bore as they should. So you are going down the road with the brakes applied lightly on the LF. This causes heat to build up that cooks the seals on the pistons, boils the brake fluid and reduces the life of the caliper. When brake fluid boils it is not as affective, so it is increasing braking distances.

I would also think about replacing all four flexible lines, the LF caliper (or you can rebuild it, if you can get the pistons out but they may be frozen in their bores) and replace the brake fluid. MB recommends changing the fluid annualy. I always use OEM fluid too. Brake fluid is hydroscopic (absorbs moisture) so when the caliper piston seals cooked, it could allow moisture between the piston and the bore then the brake system starts to rust from the inside out.

I think the chances of your brakes just failing are very small because MB uses a dual diagonal system so that if one part of the brakes fail such as the LF, you would lose the RR but still have the RF & LR. Stopping distances go up dramatically, but at least you still have hydraulic brakes and are not totally dependent on the parking brake.
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  #23  
Old 08-24-2006, 02:17 PM
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Quote:
Originally Posted by riethoven View Post
I think the chances of your brakes just failing are very small because MB uses a dual diagonal system so that if one part of the brakes fail such as the LF, you would lose the RR but still have the RF & LR. Stopping distances go up dramatically, but at least you still have hydraulic brakes and are not totally dependent on the parking brake.
The systems are divided into front and rear brakes. The forward reservoir feeds the rear section of the m/c for the front brakes. The rear reservoir feeds the front section of the m/c for the rear brakes. There is no diagonal system.

This description applies to the W126. The W123 may have a different m/c setup, but it does not have a diagonal system.
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  #24  
Old 08-24-2006, 02:25 PM
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The vehicle this happened to me in was a '77 Chevy C20 crew cab pick-up. It did not have the diagonal system as the MB. If you lose any hydraulic pressure on the rear, you have no rear brakes, except for the emergency brake. Same for the front, except you lose the front portion. Diagonal set up would have been much better, as you have proportional front rear stopping power, even if it is a partial system.

When adrenalin is pumping, you can push a brake pedal to the floor. I have found that just because something "can't happen" either on paper, or on the bench, does not mean in won't happen in the real world arena.
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70's Southern Pacific #5608 Fairmont A-4 MOW car

13 VW JSW 2.0 TDI 193K, Tuned with DPF and EGR Delete.

91 W124 300D Turbo replaced, Pressure W/G actuator installed. 210K

90 Dodge D250 5.9 Cummins/5 speed. 400K
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  #25  
Old 08-24-2006, 02:40 PM
rblookc
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I very much appreciate the input. The LF caliper piston is not stuck. I got it to move when I put the new pads in. I purchased two reman calipers to be safe. 83 bucks each not bad, and maybe the complaints about brakes will go away. I recognize the points on both sides. Probably not a problem, but there is a difference between something braking and being stuck on the side of the road, and something not braking and being off the road or welded to someone's back bumper. Keep the debate alive. Its informative. But I wanted to say thanks.
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  #26  
Old 08-24-2006, 02:42 PM
rblookc
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Oh, and by replacing the fluid you mean draining and bleeding and the whole routine, right? Did that last summer, so I am on schedule.
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  #27  
Old 08-24-2006, 03:12 PM
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OK, maybe it is VW's or BMW's

Quote:
Originally Posted by Brian Carlton View Post
The systems are divided into front and rear brakes. The forward reservoir feeds the rear section of the m/c for the front brakes. The rear reservoir feeds the front section of the m/c for the rear brakes. There is no diagonal system.

This description applies to the W126. The W123 may have a different m/c setup, but it does not have a diagonal system.
I thought that Mercedes also had diagonal systems so that you would always have at least one front wheel braking. I believe that Saab had this first.
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  #28  
Old 08-24-2006, 03:36 PM
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Quote:
Originally Posted by riethoven View Post
I thought that Mercedes also had diagonal systems so that you would always have at least one front wheel braking. I believe that Saab had this first.
I never could understand the logic of such a system. Since the front brakes do 75% of the stopping, the vehicle would pull badly toward the side with the working front caliper. The rear caliper could never match the stopping power.

In any case, the M/B does not have this system.
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  #29  
Old 08-24-2006, 03:49 PM
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Quote:
Originally Posted by Brian Carlton View Post
In any case, the M/B does not have this system.
CORRECT, speaking for a W123.
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  #30  
Old 08-24-2006, 09:26 PM
John Holmes III
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No responsible shop would let that 1985 300D out the door unless the owner signed a waiver indicating that he understood the brake system was unsafe and refused to have the system repaired. The potential liability is huge.

When the brake fluid boils one day due to a faulty brake hose and/or sticky caliper, the brake pedal will go to the floor and the vehicle will not stop. It doesn't matter if the system is dual or not. All it takes is a long highway run with a sticky caliper to boil the brake fluid in a car that drives just fine around town and doesn't pull to one side during heavy braking.

Yeah, brake pads can take alot of heat, however Dot 4 brake fluid boils at under 500 deg/f. The boiling point is reduced when it is contaminated by water, as is the case with most brake fluid that is more than a couple of years old.

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