2.3-16 scary brake moment

Discussion in 'Wheels, Tyres, Brakes & Suspension' started by neilrr, Apr 23, 2008.

  1. neilrr

    neilrr MB Club Veteran

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    Driving down the motorway the other day & making a beeline for a favourite curvy offramp, I hit the brakes quite late-ish to knock off some speed & the pedal goes nearly all the way down with very little braking going on.

    Pumped it a couple of times VERY quickly & pressure returned enough to slow the car & make the bend with no harm done & I nursed it home easily enough, but obviously something's amiss.

    The car is little used - maybe taken out for a 20 mile run once every 6-7 weeks.

    Any ideas what could cause that & what I need to do to fix it?
     
  2. television

    television MB Club Veteran

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    If there are no leaks around the callipers (easily seen) then the master cylinder could be at fault if you can pump it up,it is doubtful if it has air in the sytem as where would it come from, with air in the system, by pumping the pedal to get it up hard, just ease the pressure, and it will slowly sink again.

    Master cylinder I say
     
  3. OP
    OP
    neilrr

    neilrr MB Club Veteran

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    Thanks, PM'ed you.
     
  4. television

    television MB Club Veteran

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    When fitting another master cylinder and or when bleeding, it is best to put something under the brake pedal to stop it from going pasts its normal range of travel as this has been known to damage the piston seals when pumping the pedal

    Just a piece of wood to reduce the travel to stop the pedal from bottoming out
     
  5. MFK

    MFK Hardcore MB Enthusiast

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    Also worth replacing the fluid. I have heard of the pedal dropping to the floor like this if the fluid overheated for some reason.
     
  6. OP
    OP
    neilrr

    neilrr MB Club Veteran

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    Hi,

    Thanks. Someone has suggested it might be boiled fluid from a locked on caliper or handbrake. I'll have to take it in to the local indie as I don't feel qualified to mess around with brakes myself.
     
  7. grober

    grober MB Club Veteran

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    Brake fluid is hygroscopic i.e absorbs water. When the water content is high the fluid can boil easily leading to a soft pedal under heavy braking . see http://auto.howstuffworks.com/question451.htm

    Its recommended brake fluid "should" be changed every 2 years to avoid this.
     
  8. stevesey

    stevesey Hardcore MB Enthusiast

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    Seems unlikely to be boiled fluid if the pedal went straight to the floor and you hadn't done anything to get any heat into the brakes just beforehand (unless you have a one stuck on - should be able to feel the hot disk after a short run it that's the case).

    I have had this happen on two Astra's (one a friends) - worn seals in the master clyinder flipping, then flipping back when you pump the pedal. Hence as Malcolm I'd go for Master cylinder.
     
    Last edited: Apr 23, 2008
  9. Frank O' Phile

    Frank O' Phile Hardcore MB Enthusiast

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    boiling brake fluid


    I once managed to boil brake fluid after fitting new drums to the rear of a Triumph Acclaim I ran while impersonating a student. the new drums were a snug fit, shall we say, and to bed them in I took a run to visit a friend.

    At the end of the dual carriageway, nothing happened in the braking department. Luckily I got around the island and nursed it to said friends house where a brandy, nerve calming fag, and clean underwear were given out.


    By the time the scenario had been related and the brakes felt again, all was in order. It was a slow run home and the fluid was changed next day.

    If your merc hasnt been used for some time and moisture has had time to gather, all you need is one slightly sticky piston and the moisture will evaporate adn leave you with your foot to the floor. if your brake pedal pressure is okay now, without doing anything to it, then this is likely to be the cause and you should change the fluid.

    Keep us posted as we all all like breaking news.:)
     
  10. Dieselman

    Dieselman Banned

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    I'd go with the boiled fluid.
    It will be due to binding parking brake shoes and heat transferred into the brake fluid.
     
  11. Bazzle

    Bazzle Hardcore MB Enthusiast

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    May also be worn wheel bearings or warped disks knocking the pads back after the run.

    Bazzle
     
  12. jadefox

    jadefox Hardcore MB Enthusiast

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    Just to let you know that this happened to me in my old car - it turned out to be overheating in the end. We had fitted new brakes to the car when it went in for it's MOT and a few weeks later we were driving up the motorway, we came off on the slip road and headed for the roundabout and when my husband broke there was nothing - he managed to gear change down to a reasonable speed and then we got recovered home. When the garage looked at it they said the breaks had been tightened too much and the fluid had overheated.....no apology though - suffice to say we never went back! :rolleyes:

    Good luck finding the problem.
     

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