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Thread: 2003 A6 C5 Allroad brakes fail

  1. #1

    Question 2003 A6 C5 Allroad brakes fail

    I was travelling in traffic in my 2003 C5 Allroad when my brakes failed, pedal goes to the floor with little resistance and provides no stopping power. I use discretion and the handbrake until I find a place to get off the road, stop and park. The brake fluid level is high which suggests no brake fluid loss or major leak. I noticed no warning lights. There was no apparent loss of engine power. I would anticipate that a brake booster fail would cause an increase in the brake pedal resistance rather than a decrease.

    After 15 minutes I started the car again and the brakes returned to about half strength.
    Naturally I would like a definitive answer as to what is going on and how it can be reliably fixed.
    I have done some googling and am going back to the car today to investigate further.
    These pages described similar circumstances:
    I will need to recheck the fluid level today in daylight.

    Any ideas on this situation would be most welcome.
    2003 C5 Allroad 2.5TDI
    1996 B5 A4 1.8T (sold, still in family)

  2. #2


    News to me. Have just read the two links and at least they refer to the same issue, whereas you do not seem to have lost any fluid. This is just postulating at the moment as i haven't looked at the design of the booster but it would seem that the failure points would be the master cylinder seals or the ABS unit. There should be a mechanical link between the pedal and the master cylinder piston, assisted by the vacuum booster. So if you can depress the brake pedal fully then the piston in the master cylinder must move. The next place seems to be the ABS unit. Nothing after that comes to mind as you wouldn't get a system full of air just like that.

    Can't be any more specific than that at the moment.
    B5 RS4 : Lightly modified -- C5 2.7 allroad -- B5 A4 1.8 quattro 132 Kw -- UNIMOG 404 TLF8

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    ginkgo (April 23rd, 2018)

  4. #3
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    Blue Mountains, NSW
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    Check the master cylinder. The brakes should still work even if the ABS plays up, even if you remove the ABS unit entirely. It's designed to fail safely.

    Once upon a time when I had a 1976 VW Beetle, the brake pedal started to do exactly that. It turned out the master cylinder had pitted, and oil was simply passing the piston seals. When pressed slowly the oil would flow past the seals. But when pressed quickly the seals would work as normal. Changed the master cylinder and that was it. Life was so much simpler back then.

    2004 A6 C5 Sedan 3.0 V6 Quattro
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    1999 A4 B5 Avant 2.4 V6

    1990 100 Sedan 2.3
    1991 90 Sedan 2.3
    1999 A4 B5 Sedan 2.4 V6

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    ginkgo (May 6th, 2018)

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