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e23 Brake Pedal Falling

Discussion in 'E23 (1978-1987)' started by liv2drv, Dec 19, 2008.

    liv2drv guest

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    I don't drive my 733 very often, I took it out today and noticed the brake pedal will not hold pressure even if I pump it, it slowly starts falling to the floor. There is no evidence of leaking fluid. Is this a vacuum problem?
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    John in VA

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    az3579

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    When was the last time your system was bled? How old is the brake fluid, and are you absolutely sure you have no leaks? That sounds a lot like no brake fluid in the reservoir, or bad master cylinder...


    I wouldn't think not bleeding the system after a while would cause something this severe, only a little sponginess.
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    mooseheadm5

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    Your master cylinder is shot. It may be simply getting past the piston and/or check valve and back into the reservoir or it is leaking into the booster.
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    az3579

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    That is exactly what was happening to me. It took THREE master cylinders to figure this out! Imagine how expensive and furious I was. There is the faint possibility that the booster may be bad, or some kind of gasket of some sort that would leak the fluid in there.

    I definitely wouldn't rule out your master cylinder and would be the primary thing I would suspect if your pedal goes to the floor.
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    mooseheadm5

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    Edit, this car has a hydraulic booster, right? If the pedal is really soft, there is not a boost problem, just a master cylinder prob. If the pedal is really hard then drops down (and the brakes work well) then it is a booster system issue (usually the brake bomb.) Your description sounds like master cylinder.

    robertusmax guest

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    e28 Brake Pedal falling:

    I have a 1988 e28 M5, which I have owned since new. I am now "racing" the car in DEC events. I hung rear brakes from a 1994 540i in June 2003 (that was ok, with no deterioratiion in the b rake pedal at that thime) and then Nurburgring brakes in the front from an european spec 1995 e34 touring M5 in May 2004. Ouch! Since adding the Nurburgrings in 2005 I have had a "soft but effective" pedal. I have attempted to fix the pedal by bleeding, rebleeding, steel brake lines, rebuilding the calipers (all 4), new pads, new/resurfacing the rotors, and replacing the pressure accumulator (the bomb). Most recently I put on a new Master cylinder for the 1993 730i on the undocumented inference that its stroke volume is greater than stock (the bore diameter is 25 mm rather than 23 mm, an increase of 18% in volume if the stroke lengths are the same). This effected no change. This week I had the dealer check the hydraulic pressure distributor switch as well as the pressure accumulator, rebleed, check the lines, check all 4 calipers... I have not touched the Hydraulic brake booster which looks like a M/C and to which the M/C bolts, and which stands b etween the Power Steering Fluid and the Brake Fluid nor the brake pressure hydraulic switch... My dealer mechanic tells me they are functioning properly. I now have an intractable problem of the brake pedal feeling non-existent or soft halfway to the floor (or more), and then firm brake pedal... but oh, how heartstopping is that 1/2 second delay till effective braking... both on the road and at the track.
    Our logical conclusion is that the stroke volume from the master cylinder is too low. We think the original brakes required less volume to move the pistons than do the 540i rear b rakes + the euro spec e34 front brakes. Although ilntellectually appealing, it may still be untrue, however.
    a; Is this inference true? Data?
    b: If the Stroke Volume inference were true, does it make any difference?
    c: Where else might the fault be? I can not safely drive the car in this state.
    d: How can I fix it?
    Robert M. Shuman

    liv2drv guest

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    The fix

    My problem was the Hydro Boster. The dealer wanted almost a $1000.00 for it. I found one in a salvage yard for $63.00 and it fixed the problem.

    Needless to say I was very happy with the result. It took a little more work on my part but that just made the outcome even more satisfying..

    Thanks for the reply..:)

    robertusmax guest

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    Thank you, Dave. I think that the Hydraulic Brake Booster is the likely cuprit too, just because I have been everywhere else except here. This I have not looked at. It is also logical, and I know of no way of distinguishing its failure from failure of other components in the braking system. I was just reading about an e28M5 on the mye28.com site where he hung e34 REAR Brakes and Porsche FRONT Big Brakes from Brembo on his car, without Pedal problems, so perhaps NOT Stroke Volume, eh? I will try this. BavAuto.com does not have the Brake Hydraulic Booster, but BMPDesign does as a "brake booster". . .for $553.08! Hmmm. Perhaps a low model alternatative from lthe Rest Home for the Departed, such as your idea...

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