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M3 Soft Pedal after hard cornering/ drifting with BBK

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    M3 Soft Pedal after hard cornering/ drifting with BBK

    I am having issues with my newly acquired E46 M3. Its running 6 piston Brembo GT 355x32mm on the front and 4 piston Brembo GT 345x28mm.
    Ever since buying the car I noticed that after drifting for a longer period (like around a roundabout) the first press of the brake pedal is very soft, like an airgap has formed between the discs and pads. Otherwise when driving normally, the pedal feel is very firm.
    Also when driving on track days after hard cornering on semi slicks, the first pedal apply has a very long travel, when applied very quickly, the rear axle seems to go into EBD way before the FA even bites.
    For me all signs point to an airgap or air bubble in the front axle.

    My first thought was air in the system
    • I bled the system with a power bleeder connected to the reservoir --> this didn't make a difference
    • After that I again bled the system, but this time running the DSC bleeding procedure --> Still no difference
    If its an airgap caused by disc deflection, the wheel bearings might be at fault. So I changed both wheel bearings on the front axle.
    --> This seemed to improve the situation somewhat but the pedal travel is still quite long until it becomes firm after drifting. I haven't had the chance to test the new wheel bearings on track yet.

    Does anybody have an idea what else could be causing this issue? With the engine off the Pedal is firm and does not slowly depress, meaning the master cylinder should be ok right?

    #2
    Sounds like pad knockback due to disc deflection.

    Comment


      #3
      Agreed, but why is this happening? I know a few E46 M3 drivers running the same GT6/4 Brembo bbk without such issues...

      Comment


        #4
        Originally posted by Bro1882 View Post
        Agreed, but why is this happening? I know a few E46 M3 drivers running the same GT6/4 Brembo bbk without such issues...
        What's the delta between what you're running and they are running? Braided steel lines? MK20 vs MK60? Brake fluid? etc.

        Or are you saying everything is exactly the same between your car and their cars?
        3.91 | CMP Subframe & RTAB Bushings | SMG (Relocated & Rebuilt) | ESS Gen 3 Supercharger | Redish | Beisan | GC Coilovers & ARCAs | Imola Interior | RE Rasp | RE Diablo | Storm Motorwerks Paddles | Will ZCPM3 Shift Knob | Apex ARC-8 19x9, 19x9.5 | Sony XAV-AX5000 | BAVSOUND | CSL & 255 SMG Upgrades | Tiag | Vert w/Hardtop

        Comment


          #5
          6 piston brakes seem to suffer more with pad knock-back especially if the brake pads are worn. Probably need anti knock-back piston springs.

          Last edited by Slideways; 10-14-2025, 04:52 AM.

          Comment


            #6
            Originally posted by oceansize View Post

            What's the delta between what you're running and they are running? Braided steel lines? MK20 vs MK60? Brake fluid? etc.

            Or are you saying everything is exactly the same between your car and their cars?
            Im not sure about MK20 vs MK60. What difference would it make? The ABS is not active in this case and I am not aware of the E46 having any sort of prefill logic when DSC is switched off.
            Brake fluid is standard ATE DOT4. Shouldn't make a difference in the first apply when there is no heat in the brakes.
            I am running steel lines.

            Comment


              #7
              Originally posted by Slideways View Post
              6 piston brakes seem to suffer more with pad knock-back espeically if the brake pads are worn. Probably need anti knock-back piston springs.

              https://f87.bimmerpost.com/forums/sh....php?t=1981763
              thanks, ill check this out. I wonder why these anti knockback springs aren't standard in GT brakes....

              Comment


                #8
                Originally posted by Bro1882 View Post

                Im not sure about MK20 vs MK60. What difference would it make? The ABS is not active in this case and I am not aware of the E46 having any sort of prefill logic when DSC is switched off.
                Brake fluid is standard ATE DOT4. Shouldn't make a difference in the first apply when there is no heat in the brakes.
                I am running steel lines.
                Just trying to get you to think about what is different in your setup and theirs. If they don't have the issue, but you do, then yes something is mechanically aged out or a difference does exist between your set up and theirs. Sounds like perhaps the knockback springs are potentially that difference.

                In general people are too quick to say our setups are identical, when they aren't.
                3.91 | CMP Subframe & RTAB Bushings | SMG (Relocated & Rebuilt) | ESS Gen 3 Supercharger | Redish | Beisan | GC Coilovers & ARCAs | Imola Interior | RE Rasp | RE Diablo | Storm Motorwerks Paddles | Will ZCPM3 Shift Knob | Apex ARC-8 19x9, 19x9.5 | Sony XAV-AX5000 | BAVSOUND | CSL & 255 SMG Upgrades | Tiag | Vert w/Hardtop

                Comment


                  #9
                  Originally posted by Bro1882 View Post

                  thanks, ill check this out. I wonder why these anti knockback springs aren't standard in GT brakes....
                  They are not ideal for a street setup as they will cause a bit of drag on the brakes. Older GT kits might not have them or if they do have them, the springs are too weak for heavy track/drifting use.

                  From what I have read, (newer) AP kits have them from factory.
                  Last edited by Slideways; 10-15-2025, 09:48 PM.

                  Comment


                    #10
                    Originally posted by Slideways View Post

                    They are not ideal for a street setup as they will cause a bit of drag on the brakes. Older GT kits might not have them or if they have them, the springs are too weak for heavy track/drifting use.

                    From what I have read, (newer) AP kits have them from factory.
                    I might take the time and check if my brakes have them. Maybe some of them are broken as the kit is quite old. I have noticed that the car is really hard to push. Possibly due to the springs? If only a 1 or 2 are broken per caliper, then the drag would still be high, but the fluid would still be displaced by the few pistons where the springs are broken and cause a long pedal...

                    Comment


                      #11
                      Originally posted by Bro1882 View Post
                      Agreed, but why is this happening? I know a few E46 M3 drivers running the same GT6/4 Brembo bbk without such issues...
                      Some people drive harder than others, wheel bearing tolerances etc.
                      Build Thread:
                      https://nam3forum.com/forums/forum/m...-new-pb-at-msr

                      Comment


                        #12
                        100 percent sounds like pad knockback. Check your wheel bearings for deflection.

                        Anti knockback springs are the easy button.

                        Lightly tapping brakes with your left foot exiting the corner/drift is the real answer. Good idea to do even with anti knockback springs.

                        I have springs in my stoptech kit and still find that tapping the brakes after hard corners makes the brakes feel so much better.

                        Comment


                          #13
                          Originally posted by eacmen View Post
                          100 percent sounds like pad knockback. Check your wheel bearings for deflection.

                          Anti knockback springs are the easy button.

                          Lightly tapping brakes with your left foot exiting the corner/drift is the real answer. Good idea to do even with anti knockback springs.

                          I have springs in my stoptech kit and still find that tapping the brakes after hard corners makes the brakes feel so much better.
                          Like I said, both wheel bearings on the FA and one on the RA have already been changed with OE parts. This is my first car with solid calipers and I am just really surprised by this behavior. I have also debated selling the GT6/4 and buying Ap 5000s for the front and revert back to original RA with some good Endless pads. However, the Brembos are already registered for the car. Its quite a pain to have brake changes registered in Germany as the TÜV is very very strict.
                          Is it possible to retrofit AP springs into the Brembo calipers?

                          Comment


                            #14
                            Originally posted by Bro1882 View Post

                            Like I said, both wheel bearings on the FA and one on the RA have already been changed with OE parts. This is my first car with solid calipers and I am just really surprised by this behavior. I have also debated selling the GT6/4 and buying Ap 5000s for the front and revert back to original RA with some good Endless pads. However, the Brembos are already registered for the car. Its quite a pain to have brake changes registered in Germany as the TÜV is very very strict.
                            Is it possible to retrofit AP springs into the Brembo calipers?
                            Sorry didnt read the whole thread. I dont think a different BBK would solve the issue unless they come with springs pre-installed.

                            The anti knockback springs are fairly generic. Just need to get the right bore size for your piston sizes.

                            Need to get new square cut seals and remove the pistons to install the springs. If you have dust boots on them then you might need new dust boots as well.

                            But… you might want to build the muscle memory of tapping the pedal after high G turn so you can jump in any car.

                            Comment


                              #15
                              Originally posted by Bro1882 View Post
                              1. My first thought was air in the system
                              • I bled the system with a power bleeder connected to the reservoir --> this didn't make a difference
                              • After that I again bled the system, but this time running the DSC bleeding procedure --> Still no difference
                              2. If its an airgap caused by disc deflection, the wheel bearings might be at fault. So I changed both wheel bearings on the front axle.

                              1. Not air in the system, bc why air in there only when moving around a turn?
                              2. Wheel bearing plays: I wouldn't waste the time and $$ on the bearing before rocking the mounted wheel at 3 and 9, 12 and 6 to see if there is any plays.

                              The cause is centripetal force: it pulls some pads and piston outward, creates an air gap between pads and rotor.
                              Solution: springs to push pads to rotor, but this creates drag friction. Or lightweight pads and pistons.

                              Comment

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