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      12-02-2019, 02:57 AM   #1
si_mon
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335D Xdrive Spongy brakes after pad & disc change

Hi

I have just replaced the front discs and pads on my 2016 335d Xdrive, it has the M-Performance 380mm discs, I have replaced OEM with Brembo discs and pads.

I now have a very large amount of brake pedal travel and they feel very spongy. Any ideas on how to get rid of this and make the brake pedal tight again?

I have read other peoples responses, which include give the brakes time to bed, air bubbles in the hydraulic system, air in the ABS system etc etc. Nothing seems to have worked. I have engaged ABS on slippery roads to get it to function in order to get rid of any air, but no luck. I can't see how it can be an airlock issue as I haven't opened the hydraulic system.

Thanks for any help.
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      12-02-2019, 03:16 AM   #2
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How many miles have you driven since changing the discs and pads ? The friction surfaces need time/usage to mate; say 50 miles on open road driving (some advice says as much as 200 miles).

Once the faces of the discs and pads have been settled it's a good idea to properly bed the brakes (late at night on a quiet/remote section of straight road is a good idea) :

Warm the brakes through normal use.
Do 10 consecutive hard braking runs, 60mph to 5mph, with enough retardation to almost trigger the ABS. Do not trigger the ABS, as this can cause uneven pad transfer to the faces of the discs = brake judder.
Do not let the car come to a stop - as soon as you reach 5mph, accelerate back to 60mph and do the next hard retardation.
The discs and pads will get HOT - you should expect them to smoke a little.
After 10 retardations go for a long drive, say 20 miles, to properly cool the brakes. Plan your journey so that you won't have to stop the car - hot discs and hot pads can easily increase the pad transfer when the discs aren't rotating, especially if the car is held stationary using the footbrake, and this can cause an area of thick pad material which creates brake judder.

Triggering the ABS does not get air our of the brake system. The only way to remove air bubbles in brake fluid is to bleed the hydraulic fluid.
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      12-02-2019, 03:19 AM   #3
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Who replaced them, yourself or a garage?
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      12-02-2019, 03:36 AM   #4
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If you fitted the brakes yourself did you lightly abrade the surface and did you bleed the brakes after you changed the pads?

Pedal travel / 'feel' is normally due to air in the system. You can compress air [spongy feel] you can't compress brake fluid as easily.

Just a thought..
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      12-02-2019, 04:19 AM   #5
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It does sound to me like they haven't been bled properly tbh.
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      12-02-2019, 06:45 AM   #6
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I had this once on a car - turned out the retaining springs to the pads/calipers were fitted wrong and were pushing the piston back into calipers too much.
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      12-02-2019, 07:04 AM   #7
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If the hydraulic system has not been opened then it is not air, or at least, not air introduced as a result of the pad and disc change. It does sound as though they need bleeding but they would have needed bleeding before the pad change. Bedding the discs and pads in is a good idea and will improve the braking efficiency but it will make no difference to a spongy feel.
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      12-02-2019, 08:41 AM   #8
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Thanks for the replies.

- I replaced them myself, I have replaced many pads and discs on many different cars and not had this issue.
- the Spring retaining clips have been installed correctly.
- the brakes were not spongy before this.
- Only replaced last night, approximately ~30 miles in them now.
- I have my own method of bedding new brakes in and that is basically going down a very long hill and very gently applying the brake to gently grind and mate the surfaces together, repeated again and again. You can tell if it has worked by the disc area having the same ground sheen across all of it's face.
- I shouldn't need to bleed a brake system for just replacing pads and discs?!

I have continued researching in to this and it seems to have happened to others on different forums. Seems the fix is to reset the ABS on each wheen by using a laptop and the BMW ISTA software and connecting lead, then selecting each wheels ABS and operating the ABS pump to clear any air lock. I'm still confused as to how this happened. I will try this method then report back.

- Also, this car had new brake fluid only 5 months ago by a BMW main dealer

Last edited by si_mon; 12-02-2019 at 08:46 AM..
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      12-02-2019, 08:44 AM   #9
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Quote:
Originally Posted by si_mon View Post
I'm still confused as to how this happened.
Me too. Changing discs and pads cannot introduce any air into the system that wasn't there before.
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      12-03-2019, 04:51 AM   #10
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Can't see how sponginess can be much to do with bedding in. I'm assuming the car stops dead straight? If something was not right with how you fitted them or there is an issue with a piston it is unlikely it would be on both sides and so would be obvious but this seems very unlikely especially as you clearly know what you are doing. It's the long peddle travel that concerns me more (is it almost like you are pumping the pistons out for the first time after a change every time you apply the brakes or is it really just extra travel due to sponginess?). As aside from that it does sound like they need bleeding - all I can think was there was a small amount of air in the system from the last fluid change but was somewhere where it didn't notice in the pedal feel (can't really see how though!) and that the significant shift in fluid from the pad change has shifted the air to somewhere where it is now very noticeable. Certainly can't see how changing the pads/discs would otherwise have much to do with it... Be interested to hear what fixes it...
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      12-03-2019, 06:11 AM   #11
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Quote:
Originally Posted by si_mon View Post
Thanks for the replies.

- I replaced them myself, I have replaced many pads and discs on many different cars and not had this issue.
- the Spring retaining clips have been installed correctly.
- the brakes were not spongy before this.
- Only replaced last night, approximately ~30 miles in them now.
- I have my own method of bedding new brakes in and that is basically going down a very long hill and very gently applying the brake to gently grind and mate the surfaces together, repeated again and again. You can tell if it has worked by the disc area having the same ground sheen across all of it's face.
- I shouldn't need to bleed a brake system for just replacing pads and discs?!

I have continued researching in to this and it seems to have happened to others on different forums. Seems the fix is to reset the ABS on each wheen by using a laptop and the BMW ISTA software and connecting lead, then selecting each wheels ABS and operating the ABS pump to clear any air lock. I'm still confused as to how this happened. I will try this method then report back.

- Also, this car had new brake fluid only 5 months ago by a BMW main dealer
You didn't accidentally loosen any of the torx bolts on the rear of the calipers did you? Apparently a few people make this mistake and it introduces air into the brake fluid system.
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      12-04-2019, 08:09 AM   #12
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Reading this post with interest - I also replaced all discs and pads on my 330D not long after purchasing. I'd say it took 2-300 miles to properly bed in the new brakes, including some heavy braking to settle in the new brakes. Once bedded in however they were much better than the original, worn brakes.

I also experienced some light juddering through the pedal for a while after changing - I thought I may not have seated the discs correctly but found no significant run out on the discs. Again, this was cured by several firm applications of the brakes.
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      12-04-2019, 09:44 AM   #13
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could it be disc runout causing pad knockback?
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