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Thread: Brake Pedal Feel Issue

  1. #26
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    One major reason TIATO's brake pedal feel improved with a BBK is that the ratio of Master Cylinder displaced volume to Caliper Piston displaced volume changed. Meaning the 'leverage' of your foot is different than before. Same movement of the foot/pedal results in greater movement of the caliper piston(s) = greater brake torque. This results in a firmer pedal feel but *can* also result in less ability to modulate brake force.

    I thought the ETK used to have MC bore size listed.....but I must have imagined it.

    There are different MC's which will fit/work but I think most are smaller, not larger than the e36 M3. Non-M cars might benefit from putting an M3 MC on, it is basically sized to deal with the caliper piston diameter (stamped onto the outside of the caliper itself). Or there is always the option of aftermarket 2 circuit MC's with individual reserviors.
    Last edited by BMWParkingOnly; 05-18-2009 at 12:02 PM. Reason: Automerged Doublepost

  2. #27
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    Quote Originally Posted by csr View Post
    Maybe it's just the MC design/size problem. Are there any other (maybe bigger) MC that could fit into an E36?

    It seems like if MC can pump out quicker, then the calipers will grip sooner and the pedal will reach the firmer state sooner.

    Are there any bigger size BMW MC that will mate with an E36 booster?
    Quote Originally Posted by BMWParkingOnly View Post
    One major reason TIATO's brake pedal feel improved with a BBK is that the ratio of Master Cylinder displaced volume to Caliper Piston displaced volume changed. Meaning the 'leverage' of your foot is different than before. Same movement of the foot/pedal results in greater movement of the caliper piston(s) = greater brake torque. This results in a firmer pedal feel but *can* also result in less ability to modulate brake force.

    I thought the ETK used to have MC bore size listed.....but I must have imagined it.

    There are different MC's which will fit/work but I think most are smaller, not larger than the e36 M3. Non-M cars might benefit from putting an M3 MC on, it is basically sized to deal with the caliper piston diameter (stamped onto the outside of the caliper itself). Or there is always the option of aftermarket 2 circuit MC's with individual reserviors.
    You guys might have the answer. Unfortunately I don't think an M3 master cyl. is SpecE36-legal, but I'd love to hear if some other 325i guys try this. Doesn't help the M3 crowd much, though.

    On my E28 535i, I did upgrade the master cylinder when I upgraded to E32 750 calipers. Didn't make much difference on the street, but under track conditions it was much better.

    -tammer

  3. #28
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    My recent evaluation and measuring the pedal travel has been done in the garage with the car up on jackstands. Any dynamic element, such as wheel bearings and pad knockback are not in play. In fact, once the pad is in contact with the rotor, nothing (theoretically) is there to make the pad move back. The whole system is static, except for, of course, the pedal/brake booster/MC piston.

    As csr stated, this may just be a stackup of pedal lever and MC geometry. The pedal has to swing x-degrees so that the MC piston moves far enough to start pushing fluid through. That is why I am asking if anyone else has tried measuring so we can get an idea of what "they all do that" really means.

    Like tammer, I can't change from the stock system per class rules. I have no trouble stopping the car on track, it just sometimes causes a poor downshift and ugly cornering.

    I did find a brand new, in the box, booster from an ebay vendor for $250 shipped, which made it reasonably priced enough to swap it out. It comes with the brake pedal travel sensor already installed (which is ~$120 from Pelican). The price of the booster could be much less if they left this part off.

    Matt
    1994 325is Spec3

  4. #29
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    Floppy pedal is my secret shame too.

    I was just at the brake cleaner store (autozone) and decided to get a price on a booster. They were 207 for a reman., but it wasn't clear what brand. SO there's that for anyone brave enought to try one. I'll stick with OEM parts for my brake system.
    Brian
    SE36 #206

  5. #30
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    Quote Originally Posted by BMWParkingOnly View Post
    One major reason TIATO's brake pedal feel improved with a BBK is that the ratio of Master Cylinder displaced volume to Caliper Piston displaced volume changed. Meaning the 'leverage' of your foot is different than before. Same movement of the foot/pedal results in greater movement of the caliper piston(s) = greater brake torque. This results in a firmer pedal feel but *can* also result in less ability to modulate brake force.
    Good explanation.

    For the record, with the BBK kits I have used, my ability to modulated brake force is unaffected and I would go as far as saying that brake force modulation is much easier than before.

  6. #31
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    Quote Originally Posted by TIATO View Post
    For the record, with the BBK kits I have used, my ability to modulated brake force is unaffected and I would go as far as saying that brake force modulation is much easier than before.
    I think this is the result of good 'feel' on the part of the driver (you). Some drivers work better by feel varying the force applied to modulate, others by relative motion with variation in the movement of the pedal. Hence my notation ** around 'can'.

    Other factors which help the ability to modulate - pad compound (did you also change pads when you changed to a BBK?), brake lines (soft lines) and sliding vs. fixed caliper/caliper guide pins. A fixed caliper will usually (okay, always) have better feel than a sliding caliper riding in plastic/rubber guides.

    For reference, which BBK did you choose?

  7. #32
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    Quote Originally Posted by BMWParkingOnly View Post
    I think this is the result of good 'feel' on the part of the driver (you). Some drivers work better by feel varying the force applied to modulate, others by relative motion with variation in the movement of the pedal. Hence my notation ** around 'can'.

    Other factors which help the ability to modulate - pad compound (did you also change pads when you changed to a BBK?), brake lines (soft lines) and sliding vs. fixed caliper/caliper guide pins. A fixed caliper will usually (okay, always) have better feel than a sliding caliper riding in plastic/rubber guides.

    For reference, which BBK did you choose?
    Initially had the UUC Alcons with Hawk DTC 70/60 split and used them through 2008.

    Currently using Performance Friction with PF01 all around.

  8. #33
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    one more datapoint to add to this..

    I have had the exact same problems with my brake pedal over the last 3 years (spongy and too low for good hnt). I upgraded to a new M3 master cylinder which has a larger diameter (i think 25mm vs 23mm or something similar). No change.

    I then bought a big brake kit (4 wheel UUC Wilwood using DTC 60 pads) including new stainless brake lines. Improvement noted, but pedal still spongy and still too low.

    I then replaced all the wheel bearings with no change

    Last year I finally replaced the booster with a brand new one. (came with new check valve and sensor) Again, no change.

    Each time I've pumped many liters of fluid through the system, activated ABS to pump out any air, etc.

    I also believe that the problem has something to do with the interface between the booster and the master cylinder. For now, I tap the brakes before going into a braking zone and brake early so I don't have to push the pedal in that far, allowing me to hnt.

    I'm currently working on padding my brake pedal to bring it up a bit, but that's really just a bandaid to the real problem. I've also thought about disabling the booster. I'm convinced that my brake pedal would be nice and firm without the booster but would take a lot of getting used to.

  9. #34
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    I have been reading about how master cylinders work, and how boosters work, and i dont understand how a brake booster could cause a low pedal. If i am wrong, could someone explain?
    Jon

    98 M3 Track/Autocross/Street- AST 4100s.
    94 325is beater

  10. #35
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    Quote Originally Posted by fatboycowen View Post
    I have been reading about how master cylinders work, and how boosters work, and i dont understand how a brake booster could cause a low pedal. If i am wrong, could someone explain?

    http://news.carjunky.com/how_stuff_w...rks_c176.shtml
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  11. #36
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    Quote Originally Posted by fatboycowen View Post
    I have been reading about how master cylinders work, and how boosters work, and i dont understand how a brake booster could cause a low pedal. If i am wrong, could someone explain?
    It shouldn't, but consider that when the car is off, (ie. no vacuum) the pedal is rock hard and stays up high (and doesn't sink). When the car is running pedal gets soft & low (but doesn't sink). Car is not moving in either case, which would eliminate pad knockback or bearing issues, and I would assume eliminates ABS issues. To me it seems that either something is flexing in the booster, or the interface between the MC and booster is too loose when there is vacuum.

    I found the following thread somewhat helpful for the boost/no boost decision:

    http://forums.bimmerforums.com/forum...=brake+booster

  12. #37
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    Ok, but that doesnt answer my question at all.

    In theory (based on the diagrams i have seen) it seems that the brake booster only applies added force to the rod that goes to the master cylinder. If the engine is off, the same things happen, except the added force from the diaphragm in the booster is missing, and all force comes from the imput of the driver.

    So, if you remove the brake booster from the equation, and apply an equal force (regular foot pressure, plus whatever the booster added- by your foot) the same pedal travel should happen. In other words, if you were a gorilla and had super strong legs, you would be able to push the pedal the same distance with the force of your leg, even when the car is off.

    Does that make sense? Am i correct? If so, then, how could the brake booster cause the pedal to lower?

    Quote Originally Posted by kenster View Post
    It shouldn't, but consider that when the car is off, (ie. no vacuum) the pedal is rock hard and stays up high (and doesn't sink). When the car is running pedal gets soft & low (but doesn't sink). Car is not moving in either case, which would eliminate pad knockback or bearing issues, and I would assume eliminates ABS issues. To me it seems that either something is flexing in the booster, or the interface between the MC and booster is too loose when there is vacuum.

    I found the following thread somewhat helpful for the boost/no boost decision:

    http://forums.bimmerforums.com/forum...=brake+booster
    Ok, i understand this. However, when the car is off, and you step on the brake pedal, the imput force to the MC is considerably less, because the booster isnt helping. Therefore, the deflection of the pedal would be less, and the force on the brake pads would be less (you wouldnt stop as fast).

    So, in theory, if you want the MC to see the same force when the car is off, you push the hell out of the pedal, and it should deflect the same amount as it did when the booster was working.
    Last edited by fatboycowen; 05-19-2009 at 04:26 PM. Reason: Automerged Doublepost
    Jon

    98 M3 Track/Autocross/Street- AST 4100s.
    94 325is beater

  13. #38
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    Quote Originally Posted by fatboycowen View Post
    Ok, i understand this. However, when the car is off, and you step on the brake pedal, the imput force to the MC is considerably less, because the booster isnt helping. Therefore, the deflection of the pedal would be less, and the force on the brake pads would be less (you wouldnt stop as fast).

    So, in theory, if you want the MC to see the same force when the car is off, you push the hell out of the pedal, and it should deflect the same amount as it did when the booster was working.
    I'm glad you brought that up. I'm pressing the pedal CONSIDERABLY harder when the car is off. To the point that I think something will either break or bend. The pedal goes to a point and then stops no matter how hard you press. It feels like the brake pedal is connected directly to the master cylinder. With the boosted setup on the other hand, it seems that there is a marshmellow between the brake pedal and the master cylinder. I think that's the best way to describe it.

    I suppose I could hook up a brake pressure gauge to one of my calipers to make sure I get the same pressure for both boosted and non boosted, but I'm pretty sure I'm pressing it hard enough if not harder.

  14. #39
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    Maybe someone could manufacture a Short Brake Kit (SBK) by changing the pedal design and the connection point to change the ratio. (Sort of like SSK for transmission).
    Mr. Trashy is gone... Mr. Ghost is here!


  15. #40
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    The reason the pedal feels better with no boost may be because your foot is having to compress the springs in the master cylinder and overcome any friction in the piston seals.

    Based on the thread kenster posted a link to, this may be why you still have to push pretty hard to stop the car in your driveway. The pedal feels better because you can feel the resistance of the piston springs, but system hasn't really built up any hydraulic pressure in the lines yet.

    With the booster, this inital force is greatly reduced, and you get the spongy feel until you are really into the brakes and the pressure builds up enough in the brakes lines to be greater than the boosted pressure.

    I found this pretty useful: http://www.autoshop101.com/forms/brake02.pdf

    Based on it, I am wondering if the problem is due to the distance the MC piston has to travel to close off the compensating port and push the stem on the valve of the secondary piston before the hydraulic pressure can build up in the chambers (I am assuming the BMW, with only ABS, has a portless secondary and conventional primary master cylinder-page 18).

    For now, I am pretty much convinced this is how the system works and this is not a result of a bad MC, wheel bearings, ABS pump, booster, SS lines, etc.

    Matt
    1994 325is Spec3

  16. #41
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    try unhooking the vacuum line from the intake manifold to the booster. pedal effort goes up but feel is greatly improved. been running this way for years. remember to plug the vacuum line. just be ready on that first stop to press harder. but once you are used to it you dont notice anymore.
    Ryan
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  17. #42
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    Quote Originally Posted by Specter325 View Post
    try unhooking the vacuum line from the intake manifold to the booster. pedal effort goes up but feel is greatly improved. been running this way for years. remember to plug the vacuum line. just be ready on that first stop to press harder. but once you are used to it you dont notice anymore.
    that's exactly what I plan on doing at my next event. My only concern is that I use the foot roll version of heel n toe. I hope I'll still be able to use this method for downshifts with the additional brake pressure required to stop the car.

    Quote Originally Posted by matt325 View Post
    The reason the pedal feels better with no boost may be because your foot is having to compress the springs in the master cylinder and overcome any friction in the piston seals.

    [snip]

    I found this pretty useful: http://www.autoshop101.com/forms/brake02.pdf

    Based on it, I am wondering if the problem is due to the distance the MC piston has to travel to close off the compensating port and push the stem on the valve of the secondary piston before the hydraulic pressure can build up in the chambers (I am assuming the BMW, with only ABS, has a portless secondary and conventional primary master cylinder-page 18).

    For now, I am pretty much convinced this is how the system works and this is not a result of a bad MC, wheel bearings, ABS pump, booster, SS lines, etc.
    nice informative article. I don't think the spring pressure inside the master cylinder plays much of a factor however. When bench bleeding the master cylinder, I can easily push the piston in by hand compressing the internal springs.
    Last edited by kenster; 05-20-2009 at 06:34 AM. Reason: Automerged Doublepost

  18. #43
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    Quote Originally Posted by matt325 View Post
    The reason the pedal feels better with no boost may be because your foot is having to compress the springs in the master cylinder and overcome any friction in the piston seals.

    Based on the thread kenster posted a link to, this may be why you still have to push pretty hard to stop the car in your driveway. The pedal feels better because you can feel the resistance of the piston springs, but system hasn't really built up any hydraulic pressure in the lines yet.

    With the booster, this inital force is greatly reduced, and you get the spongy feel until you are really into the brakes and the pressure builds up enough in the brakes lines to be greater than the boosted pressure.

    I found this pretty useful: http://www.autoshop101.com/forms/brake02.pdf

    Based on it, I am wondering if the problem is due to the distance the MC piston has to travel to close off the compensating port and push the stem on the valve of the secondary piston before the hydraulic pressure can build up in the chambers (I am assuming the BMW, with only ABS, has a portless secondary and conventional primary master cylinder-page 18).

    For now, I am pretty much convinced this is how the system works and this is not a result of a bad MC, wheel bearings, ABS pump, booster, SS lines, etc.
    When reading up on this, one page was saying that you can easily calculate the pressure provided by the booster. It gave an example of an 8 inch booster, which gives about 250 pounds of force added to your foot force. Thats a pretty big number.
    Jon

    98 M3 Track/Autocross/Street- AST 4100s.
    94 325is beater

  19. #44
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    Just bumping this up to report some recent experiences with my system. I was at Mid-Ohio this past weekend. Fresh Ate 200 brake fluid (2 days before the event), started on 1/2-worn Hawk HT-10 front and Porterfield R4 rear. Brakes worked great, but pedal was low and spongy (the norm). On day 2, I replaced the front pads with Hawk HT-14. The pedal was perfect for one or two sessions ... high, firm, exactly as it should be. By the third session, it was right back to where it started, low and spongy. Not a gradual drop-off, either, it just got worse. The new pads still looked pretty new and are plenty thick.

    I have to give this a big WTF? It can't be a problem with the component set as suggested in this thread; a friend's E36 318i (same booster, MC, calipers) has a perfectly firm pedal. Granted his straightaway speed isn't rocket-fast, but the brakes stand up to repeated abuse. It's not the MC unless we are all buying new, DOA master cylinders. It's certainly not the calipers or pad compound. BTW, I replaced the front rotors later on Saturday and the rear pads on Sunday; both of those did nothing to the pedal feel/travel.

    Components I haven't replaced:
    *booster
    *ABS pump

    I can't think of a logical explanation for a booster failure to cause this type of condition. The pedal is firm when left-foot braking and on-throttle (less vacuum in the system, less boost). Is the stock setup just over-boosted? Could the ABS pump somehow contribute to this?

    -tammer

  20. #45
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    If the stock setup is over-boosted, why is it only the case for some of us? Also, as I previously said, I replaced the booster and ABS pump as well with no change in feel. I guess it cant hurt to have a completely replaced brake setup...

  21. #46
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    Quote Originally Posted by VengeanceM3 View Post
    If the stock setup is over-boosted, why is it only the case for some of us? Also, as I previously said, I replaced the booster and ABS pump as well with no change in feel. I guess it cant hurt to have a completely replaced brake setup...
    Well, I have a plan (and that's a dangerous thing).

    For my next event, I will do a few things:
    1) Replace fluid with Motul. I did try bleeding the brakes when I swapped pads at MO; everything looked peachy, no air, fluid looked and smelled new. I'll drive it on the same pads and current setup with the different fluid.

    2) Change booster vacuum line, check valve, and clamps. Drive.

    3) Move to a completely different pad compound, either a Pagid or PFC. Drive.

    That should keep me reasonably busy in the paddock, but I don't want to just throw a whole bunch of parts at it at once. If none of that changes it, the last component in the system is the ABS pump. I doubt that's contributing, because when I do get into the ABS the pedal kicks back some. But we shall see.

    -tammer

  22. #47
    NeilM is offline Member BMW E36 M3 Expert
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    Quote Originally Posted by tammer View Post
    On day 2, I replaced the front pads with Hawk HT-14. The pedal was perfect for one or two sessions ... high, firm, exactly as it should be. By the third session, it was right back to where it started, low and spongy. Not a gradual drop-off, either, it just got worse. The new pads still looked pretty new and are plenty thick.
    Hey Tammer, that makes it sound as if your brake caliper pistons aren't self-adjusting properly to take up pad wear clearance.

    Neil

  23. #48
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    Quote Originally Posted by NeilM View Post
    Hey Tammer, that makes it sound as if your brake caliper pistons aren't self-adjusting properly to take up pad wear clearance.

    Neil
    As if they are then retracting after release? Hmmm......that's an interesting thought. Though after replacing both F/R with new pads this should be much less of issue, if any. IIRC the car exhibited the same feel with new or very worn pads. That's how I recall it being.

    Cheers.
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  24. #49
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    Quote Originally Posted by RacerX View Post
    As if they are then retracting after release? Hmmm......that's an interesting thought. Though after replacing both F/R with new pads this should be much less of issue, if any. IIRC the car exhibited the same feel with new or very worn pads. That's how I recall it being.

    Cheers.
    Just to clarify, Jack shared my car at Mid-Ohio this past weekend. The feel was good for a session or so after putting new pads on, but that's it. The piston retracts enough that the wheel will spin freely when jacked up, but I thought that is normal, healthy behavior (no pad dragging/caliper binding). Pad wear is even top-to-bottom, and the fronts do wear on a slight bias (the inner and outer used pads can be stacked together with parallel backing plates, but a slight diagonal mating face, like this: |/| This is with 3-event-old front wheel bearings. Rears wear perfectly evenly in all directions.

    -tammer

  25. #50
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    Tammer,

    My stock brakes perform similarly to yours, great with fresh pads, gets much worse as they wear. Pad knock back and/or tapered pad wear would explain the lowered pedal. I also suspect that as the piston and slider pins get extended there, is more "slop" in the system (especially when the pads start to taper) which contibute to the "feel" issue. I.e. some of the force exerted by the hydraulic system is being taken up by deflection of the caliper, piston and backing plate rather than just exerting pressure on the pad. Probably less of an issues with new pads because there is less cantilever on the piston and slider pins and now pad taper (no where to go but in).

    I have brass bushing which have been on the car for ~5 years, so it may be time to replace those. Although my next step will probably be a front wheel BBK.
    Shea Burns

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