Question about Mercedes-Benz E-Class

2 Answers

W124 E280 1993: Brakes only work at bottom of pedal travel and then suddenly grab. Master cylinder has changed twice but there is no change. Two workshops have tried to find solution with no answers.

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  • philbaldwin Jul 24, 2009

    That response was too general about brake problems. I have a specific problem that replicates the symptoms of a failure of the primary piston in that it has no effect and then when the secondary piston comes into action the brakes suddenly operate, suddenly. Two workshops have spent many hours fitting new master cylinders and also a new vacuum servo unit, none of which cured the problem. Experienced people are left scratching their heads, which is why I am looking for someone who has seen this problem before. The ABS is working. Thank you.

  • philbaldwin Jul 24, 2009

    Thank you -

    I think that sounds right to me, you described correctly what it feels like when braking. I have just asked my engineer was scratching his head yesterday, and he said that he couldn't find the compensating valve, and wonders if it is part of the ABS system?

  • philbaldwin Jul 24, 2009

    Actually, the car is a Mercedes E280 (W124), but that's a good idea, I'll ask a dealer - I'm unable to find any references to compensating valves or brake pressure regulators for this type of car on the internet. It's all a bit of a mystery!

  • philbaldwin Jul 24, 2009

    I've had a conversation with a Mercedes dealer who says that there is no compensation valve on this model even though there is an ABS system. He suggests disconnecting the ABS to see if it is causing the problem - i.e, if when disconnected it causes the car to brake normally then the ABS has a fault. What do you think?

  • philbaldwin Jul 24, 2009

    Thank you for that info. I've went out and unplugged the ABS, but it makes no difference to the way the brakes work. Then it rained so I went out and drove down a hill and braked quite sharply (not a stamp!) and the ABS worked perfectly. So I think we have eliminated that. The one-way valve from the engine to the vacuum servo unit has now been checked so it seems that is OK too.

    There is now a theory that after the vacuum servo unit and the master cylinder was replaced maybe the brake fluid bleeding process was not done properly or needs doing again for some reason so I am arranging to have that done on Monday. I think its all about eliminating things now.

  • philbaldwin Jul 24, 2009

    Thanks for all the ideas, I'll come back again if the problem is still not solved on Monday.

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  • Master
  • 1,185 Answers

More out of interest than anything else

Does the car slow down with the feeling that you are sinking , ie does it tend to press you down as if you are suffering the effect of heavy gravity.

When they kick in do you then get a nosedive action.

We did a lot of work on Rally cars and one problem we had was that an adjustable valve to allow you to apply the brake pressure to the front or rear of the car depending on the stage went sick

The result was that the car tried to press its self down into the road under braking

All cars have a similar automatic system installed to split the braking on a ratio of 60/40 or even more depending on the make.

This has nothing to do with ABS or traction control it it there to stop the car in a controlled manner by giving the front brakes priority and to prevent the back brakes locking up and flipping you sideways.

In a 93 model motor it is possible that the valve is staying open and that you are getting what would be the response of an emergency stop following rear brake failure

Posted on Jul 24, 2009

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  • Tom
    Tom Jul 24, 2009

    As far as I am aware and unless someone in Germany has become very devious it should be a stand alone device which is normally located in the line to the rear brakes before it splits into the dual braking lines.



    A call to any of the BMW parts departments should confirm that as they will be able to call the exploded drawing up on the screen.



  • Tom
    Tom Jul 24, 2009

    For the ABS to work it it needs to sense a lack of rotation one a wheel it then momenterily switches off the pressure to that wheel to allows it to rotateand will keep on doing so until the rotation takes place.



    You can switch off the ABS very easily by un-plugging one of the wheel sensors which will cause the EMU to revert back to a normal braking system.



    (If the ABS is acting up then I would have expected a warning light on the dash telling me that it was having a headache and if you unplug a sensor that light should appear)



    So it 15 minutes you can either prove or disprove the ABS theory



    It still leaves a doubt in my mind that should the ABS fail and you have to ***** on the brakes the car is then in 50/50 brake balance mode which will flip you sideways as the back locks up.



  • Tom
    Tom Jul 24, 2009

    Sorry in my last response I used an English word meaning a vagrant which in America means a lady of loose morals.



    I should have said "Stamp" on the brakes

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  • Master
  • 601 Answers

First of all, most rotors and drums rust and that is not a concern.
As to the difficulty in braking, I would guess from the symptoms that is is the master cylinder, or the power assist, or an adjustment between the master cylinder and the power assist.
Also, you can easily eliminate the wheel cylinders or calipers by visually inspecting them from the outside. If you see dampness (usually accompanied by a lot of grime and brake dust trapped in it) on any of the wheel cylinders or calipers, then look there first. If those are clean, go back to the master cylinder and the power assist unit

Hope this helps
Let me know
Thanks and rate
Smith

Posted on Jul 24, 2009

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Did you keep the brake fluid reservoir from going dry while bleeding? If it went real low, air might have got back into the lines.

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