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BMW 3-Series (E90 E92) Forum > E90 / E92 / E93 3-series Technical Forums > Suspension | Brakes | Chassis > Sinking pedal after pad/rotor change



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      01-19-2018, 11:27 AM   #1
taibanl
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Sinking pedal after pad/rotor change

Due to an impending deployment I had a reputable shop replace my pads and rotors which normally I would do.

Upon getting into the car I immediately noticed a soft/squishy pedal response which I pumped up only to have the pressure gradually subside over the next 20-30 seconds. I pulled onto the road and immediately back in the parking lot where I continued to pump on and off for 5 minutes with no overall change. Keeping mild to moderate pressure on the pedal will drive the floor over about 30 seconds. The shop then manually re-bled with no change.

After the mechanic and the owner tested the car, they say the brake response is perfect.

I did change to akebono pads which I've read have a different pedal feel (smoother/gradual bite) but this should be on the road, not at a standstill, correct?

At a standstill my thought is that a non compressible fluid against a ceramic pad and metal rotor should not allow any additional travel after firm pad to rotor engagement.

Based on my understanding and additional research, the only conclusion is that the master cylinder was damaged, either by improper retraction forcing fluid past the MC seals, or over extension in one log pedal push after the pad change which can force the piston into a corroded/rough area of the MC that doesn't often see piston travel (i.e. beyond normal range of travel) thus cutting/abrading the seals.

Im personally guessing the first.

Is my conclusion correct that this is definitely a MC problem? Theres no way that ceramic akebono pads could behave like this right?
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      01-19-2018, 01:58 PM   #2
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Hey Taibanl,

From what you described, your thought is correct. After swapping pads/rotors and bleeding you may initially have to pump the pedal a couple times to push everything together, but once the pedal is firm it should maintain constant pressure. That doesn't mean that you can't push the pedal further to the floor, but doing so will be met with a linear increase in pedal pressure.

What you're describing sounds to me like air in the lines, and just like when people complain about 'warped' rotors or a pulsating pedal, our first piece of advice is always to try the easy stuff again (in this case re-bleed, for the rotor issues: re-bed).

So, before deciding the MC is the problem, I would manually re-re-re-bleed the system (with the engine running, without a power bleeder) while tapping the calipers with a rubber mallet to be SURE there isn't any air left in there. We've never had this issue with our Sparta calipers, but I can remember one particular set of OE calipers that I had to bleed 4 times, drive it around for a day, then bleed one last time before the last few bubbles came out. It could also be as simple as a little human error letting the MC res run dry for a split second, thus letting a little air into the top of the system, which could mean having to flush the entire system before you get that air back out.

Good luck!
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      01-19-2018, 02:13 PM   #3
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Thank you.

Shop is intending to do a complete reflush but effectively denying any possibility of a more serious problem.

Lets hope its air.
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      01-21-2018, 08:59 AM   #4
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Quote:
Originally Posted by taibanl View Post
Thank you.

Shop is intending to do a complete reflush but effectively denying any possibility of a more serious problem.

Lets hope its air.
I wonder if they bleed the lines correctly? I know that sounds silly.

FYI -

The Bentley Repair manual has the person opening the valve first and the person in the car pumping 8-10x and then holding the pedal to the floor. When the pedal is held to the floor, you then shut the valve. They should have done the passenger rear, driver rear, passenger front & driver front, in that order. Each wheel should be done 3x, and the mechanic should have used DOT 4 fluid.

Most people bed in this manner -

Drive the car to 60mph and press firmly/hard on the pedal without setting off the ABS system to about 15mph. Do this 8-10x in a row.

You might have air in the ABS system, if they re-bleed the system, and it still doesn't feel right.

Also, keep in mind Akebono pads will take 500-1000 miles to fully break in and get the bite, as they say.

Also, as with any new set of pads and rotors, try really, really hard not to set off the ABS system in the first 500 Miles after install.
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      01-21-2018, 09:37 AM   #5
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Quote:
Originally Posted by mweisdorfer View Post
Quote:
Originally Posted by taibanl View Post
Thank you.

Shop is intending to do a complete reflush but effectively denying any possibility of a more serious problem.

Lets hope its air.
I wonder if they bleed the lines correctly? I know that sounds silly.

FYI -

The Bentley Repair manual has the person opening the valve first and the person in the car pumping 8-10x and then holding the pedal to the floor. When the pedal is held to the floor, you then shut the valve. They should have done the passenger rear, driver rear, passenger front & driver front, in that order. Each wheel should be done 3x, and the mechanic should have used DOT 4 fluid.

Most people bed in this manner -

Drive the car to 60mph and press firmly/hard on the pedal without setting off the ABS system to about 15mph. Do this 8-10x in a row.

You might have air in the ABS system, if they re-bleed the system, and it still doesn't feel right.

Also, keep in mind Akebono pads will take 500-1000 miles to fully break in and get the bite, as they say.

Also, as with any new set of pads and rotors, try really, really hard not to set off the ABS system in the first 500 Miles after install.
Thank you.

Although I fully expect the need to bed I'm certain thats not at issue here since this is happening static (parked)

I didn't hear from them on Friday which I would've expected to had they solved with a re-flush/re-bleed.

Fingers crossed they haven't destroyed the MC.
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      01-23-2018, 10:35 AM   #6
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See post #12 in a thread I created:

http://www.e90post.com/forums/showthread.php?t=1421826

Is this what you are referring too? I wasted almost $1000 swapping my MC trying to fix this problem and had no change in pedal feel. I've bled the brakes many times, with activating the ABS module, and it still feels like this.

Last edited by Volasko; 01-24-2018 at 11:40 AM.
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      01-28-2018, 10:05 PM   #7
taibanl
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Quote:
Originally Posted by Volasko View Post
See post #12 in a thread I created:

http://www.e90post.com/forums/showthread.php?t=1421826

Is this what you are referring too? I wasted almost $1000 swapping my MC trying to fix this problem and had no change in pedal feel. I've bled the brakes many times, with activating the ABS module, and it still feels like this.
Yes. Seems similar. So your car is still like that? Did you switch to akebono at this chsnge?
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      01-29-2018, 05:42 PM   #8
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Quote:
Originally Posted by taibanl View Post
Yes. Seems similar. So your car is still like that? Did you switch to akebono at this chsnge?
When I took the video I had Akebono pads that where pretty much done from a track day. Swapping the pads to centric posi quiet ceramic pads helped a little but not that much.
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      02-03-2018, 06:01 AM   #9
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I know this might sound obvious but have the brakes been bedded in yet? It would be almost negligent for a shop to handover a car with useless brakes.

Whenever the pistons have been pushed back to fit new pads, the pedal has to be operated to push the pistons out again to take up the gap. After this, with the engine running, the pedal will feel soft and sink further than normal for many strokes. You will have to pump it and hold the pedal with firm pressure about 5-10 times and it will come good.

I've heard of one story at work however, where brake pads were replaced on a Mini and the pedal had the same symptoms as yours. Solution was to pump the pedal as if one were manually bleeding brakes, hold the pedal down firmly and crack the union nuts at the DSC inlet (do one, tighten it then return the pedal, pump, hold and crack the second). Then pressure bleed and then bleed the DSC module using the scanner to activate valves and pump.
One of those rare phenomenons..
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