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Thread: Too Much Brake Pedal Travel

                  
   
  1. #1

    Too Much Brake Pedal Travel

    I would appreciate some advice on how to firm up the brake pedal on my 1998 M-3. It seems to work all right under normal street driving conditions but, at the track, under maximum braking, the pedal goes almost to the floor with very little braking effect then, after a pause, it pushes back and braking begins in earnest. In fact, after an initial delay, holding a constant pedal pressure typically produces over-braking due to the push-back. I assume the push-back is produced by the ABS although it is not associated with wheel lock-up, but merely hard braking, as I am nowhere near “lock-up” and the push-back is not accompanied by ABS “stutter”. The delay in obtaining maximum braking effect is very disruptive to competitive driving as well as unsafe, since last minute braking leaves me wondering whether I will make the turn at all. However, as I said, with the push-back, I typically find that the car has slowed more than necessary by the time I get to the turn-in point. This problem seems to be rather common among E-36 drivers I talk to at the track. I have tried to address the problem with 4 piston calipers, oversized rotors, steel braided lines, high performance brake fluid (Racing Blue) and competition pads (Hawk Blue). None of these seem to have affected the pedal travel. I have had the brakes bleed repeatedly, including having the BMW dealer recycle the ABS control unit more than once. Someone at the track suggested that the problem may be brake pad "back lash". His explanation was that loose wheel bearings allow so much play that wheel wobble pushes the brake pads back into the calipers, especially during turning. While the bearings may be worn because the car has 90,000 miles, the brake pedal problem has been consistent since I got the car. His recommendation was to tap the brake pedal lightly to reset the pads against the rotors just before hard braking. I tried this and it seems to have the effect of bringing the pedal up tighter and making it firmer but I don't think this is because the pads have been pushed away from the rotors because when I use a light pedal application, the brake “feel” is immediate without any lag. It is just when I press hard that the braking effect does not increase proportionately. In any case, this technique of tapping the pedal before hard braking is very distracting under extreme driving conditions when I am trying to find the proper braking and turn-in points. It also causes high anxiety among other drivers who are following me when they see my brake lights blink at strange times. A BMW technician suggested that the pedal response is "normal" for ABS brakes which are designed with a pedal level senor. He said I could test whether the problem is related to the ABS by pulling the ABS fuse out but also cautioned that this would upset the front-to-rear brake bias causing the rear brakes to lock up prematurely. Whether or not this is "normal", I would like to find some way to firm up the brake pedal. Any advice you can offer about what the problem is and/or how to address it will be greatly appreciated. Thanks.

  2. #2
    Senior Member
    mlytle's Avatar
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    Quote Originally Posted by Keith1998M3 View Post
    I would appreciate some advice on how to firm up the brake pedal on my 1998 M-3. It seems to work all right under normal street driving conditions but, at the track, under maximum braking, the pedal goes almost to the floor with very little braking effect then, after a pause, it pushes back and braking begins in earnest. In fact, after an initial delay, holding a constant pedal pressure typically produces over-braking due to the push-back. I assume the push-back is produced by the ABS although it is not associated with wheel lock-up, but merely hard braking, as I am nowhere near “lock-up” and the push-back is not accompanied by ABS “stutter”. The delay in obtaining maximum braking effect is very disruptive to competitive driving as well as unsafe, since last minute braking leaves me wondering whether I will make the turn at all. However, as I said, with the push-back, I typically find that the car has slowed more than necessary by the time I get to the turn-in point. This problem seems to be rather common among E-36 drivers I talk to at the track. I have tried to address the problem with 4 piston calipers, oversized rotors, steel braided lines, high performance brake fluid (Racing Blue) and competition pads (Hawk Blue). None of these seem to have affected the pedal travel. I have had the brakes bleed repeatedly, including having the BMW dealer recycle the ABS control unit more than once. Someone at the track suggested that the problem may be brake pad "back lash". His explanation was that loose wheel bearings allow so much play that wheel wobble pushes the brake pads back into the calipers, especially during turning. While the bearings may be worn because the car has 90,000 miles, the brake pedal problem has been consistent since I got the car. His recommendation was to tap the brake pedal lightly to reset the pads against the rotors just before hard braking. I tried this and it seems to have the effect of bringing the pedal up tighter and making it firmer but I don't think this is because the pads have been pushed away from the rotors because when I use a light pedal application, the brake “feel” is immediate without any lag. It is just when I press hard that the braking effect does not increase proportionately. In any case, this technique of tapping the pedal before hard braking is very distracting under extreme driving conditions when I am trying to find the proper braking and turn-in points. It also causes high anxiety among other drivers who are following me when they see my brake lights blink at strange times. A BMW technician suggested that the pedal response is "normal" for ABS brakes which are designed with a pedal level senor. He said I could test whether the problem is related to the ABS by pulling the ABS fuse out but also cautioned that this would upset the front-to-rear brake bias causing the rear brakes to lock up prematurely. Whether or not this is "normal", I would like to find some way to firm up the brake pedal. Any advice you can offer about what the problem is and/or how to address it will be greatly appreciated. Thanks.
    first thing to do is check the front wheel bearings. at 90k they are due and some miniscule wobble will produce the symptoms you are getting.

    next, pull the abs fuse and try braking hard. it will not screw up the brake balance dangerously. you will be able to lock all the wheels though, be careful if you have never hit the binders hard w/o abs.

    next - your brake booster could be leaking. try pulling and pluging the big vacuum line from the intake to the booster. pedal should be VERY hard under braking of all kinds.

    last - your master cyl could be toast. when you jam the brakes hard, fluid is going around the seals.

    marshall
    97 m3 street car
    93 325is race car (abs removed)

  3. #3
    it's odd that you said a BBK did not solve the issue, usually that almost always does.

    Im still running stock brakes on my racecar... using Prospeed GS610 and Carbotech pads. That firms it up quite a bit. I also replaced my brake booster and it feels pretty good.
    Wonger
    #199 Salazar Blue/White IP/GTS3 M3
    http://www.brakeswap.com
    http://www.roadracetech.com

  4. #4
    Senior Member
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    Quote Originally Posted by magnetic1 View Post
    it's odd that you said a BBK did not solve the issue, usually that almost always does.

    Im still running stock brakes on my racecar... using Prospeed GS610 and Carbotech pads. That firms it up quite a bit. I also replaced my brake booster and it feels pretty good.
    actually, bbks can be the cause of long pedals. if the bbk calipers have higher fluid volume than stock and you keep the stock master cyl, then a longer pedal travel will be needed to get the brakes to work.

    i run the stock brake system on my race car too, with pf97/pf01 pads. bbk's are really window dressing for e36's. the stock brake system with stock rotors is more than up to the task of track work, including racing as you and i know. now if you drop a 350hp monster motor in the car and do long races, then that is a completely different story..

  5. #5
    Senior Member odortiz's Avatar
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    http://www.stoptech.com/tech_info/wp_knockback.shtml

    read about pad knockback. i had it bad with a BBK. more so on hard right-handers. replaced hubs. problem gone. sometimes i still, for psychological reasons, come to a corner from a long straight, while still at WOT, tap the brake pedal with my left foot before it's needed. the taps are so slight you can't feel the brakes come on. this removes any space between rotor face and pads so the whole pedal stroke is used for braking.
    95 M3, 6-speed, 3.73
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