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Old 01-04-2018, 05:47 PM
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kiznarsh
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Default Strange Brake Pedal Behavior

I bled my 981CS brakes for the first time using a pressure bleeder about a month ago and definitely got rid of some air bubbles; the pedal feel is much firmer. But now I have a strange pedal feel only when driving below 10 mph. The best way to describe it is like the feeling when you step on walnut shells and it crunches underneath you. I can also hear the pedal for the first inch of travel, whereas it was quiet before. It kinda sounds like a hand pump you'd use for your bike.

I was having a little issue with the outer bleeder screw on the front right caliper: I wasn't getting enough fluid coming out so I loosened it some more, but then fluid started coming out from the threads that are tapped into the caliper. Pressure was around 20 psi so that should've been enough, but since it wasn't, my friend stepped on the brake pedal and that helped to push it along.

I don't see how that could cause this but that's the only "non-standard" thing we did. It feels like there's air in the system at low speeds up to 10 mph, but it's absolutely fine at high speeds.

I went out and locked them up a few times last night to activate the ABS pump, and it may have gotten a little better, but it's not completely gone.

Any thoughts?
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Old 01-04-2018, 05:52 PM
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Bleed it again, old school method. Skip the pressure bleeder and get someone to work the brake pedal for you.
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Old 01-04-2018, 06:19 PM
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Originally Posted by kiznarsh View Post
I bled my 981CS brakes for the first time using a pressure bleeder about a month ago and definitely got rid of some air bubbles; the pedal feel is much firmer. But now I have a strange pedal feel only when driving below 10 mph. The best way to describe it is like the feeling when you step on walnut shells and it crunches underneath you. I can also hear the pedal for the first inch of travel, whereas it was quiet before. It kinda sounds like a hand pump you'd use for your bike.

I was having a little issue with the outer bleeder screw on the front right caliper: I wasn't getting enough fluid coming out so I loosened it some more, but then fluid started coming out from the threads that are tapped into the caliper. Pressure was around 20 psi so that should've been enough, but since it wasn't, my friend stepped on the brake pedal and that helped to push it along.

I don't see how that could cause this but that's the only "non-standard" thing we did. It feels like there's air in the system at low speeds up to 10 mph, but it's absolutely fine at high speeds.

I went out and locked them up a few times last night to activate the ABS pump, and it may have gotten a little better, but it's not completely gone.

Any thoughts?
Some times the crunching sound is either rust build up on part of the disc usually top edge or bottom edge of disc, or a worn disc where the metal part of thr pad comes in contact with the disc before or at the same time as the friction part of the pad. Thoroughly check all pads and discs both sides, easy to miss this on inside back.
Or badly scored discs can make similar sound.
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Old 01-04-2018, 06:38 PM
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Originally Posted by Russ21 View Post
Some times the crunching sound is either rust build up on part of the disc usually top edge or bottom edge of disc, or a worn disc where the metal part of thr pad comes in contact with the disc before or at the same time as the friction part of the pad. Thoroughly check all pads and discs both sides, easy to miss this on inside back.
Or badly scored discs can make similar sound.

Another crunchy scraping noise could be from pads going hard/glazed you can stop this noise by reversing fastish and braking hard a few times
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Old 01-04-2018, 07:22 PM
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Originally Posted by Russ21 View Post
Some times the crunching sound is either rust build up on part of the disc usually top edge or bottom edge of disc, or a worn disc where the metal part of the pad comes in contact with the disc before or at the same time as the friction part of the pad. Thoroughly check all pads and discs both sides, easy to miss this on inside back. Or badly scored discs can make similar sound.
Originally Posted by Russ21 View Post
Another crunchy scraping noise could be from pads going hard/glazed you can stop this noise by reversing fastish and braking hard a few times
I'm only at 10k miles and I did look at my rotors but nothing stood out. Seems like any issues due to worn or rust build-up would come on gradually, right? Yet this pretty much happened after the bleeding procedure.

A couple other people mentioned the sound could be coming from the booster. Maybe some fluid got into the booster?
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Old 01-05-2018, 01:46 AM
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Took a closer tonight and the sound is definitely coming from this bellows assembly. I pumped the brakes a few times with my hand and I could clearly hear it coming from there. Since the car was off there was no vacuum on the backside of the booster and so once the bellows stopped being compressed, the noise went away.



Made an appointment for tomorrow to see if it can be replaced/repaired.
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Old 01-11-2018, 04:00 AM
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Just to close this out, the dealer diagnosed the problem to be the brake booster. They discovered the noise coming from there and decided to replace it after no luck with lubricating the pushrod/bellows assembly.

Part was expedited from the Motherland and I picked up the car today after driving a base Macan as a loaner for 5 days. For a CUV, it actually wasn't too bad. It's essentially the same instrument cluster so it was a familiar setting, but I felt like I could see the earth's curvature from being so high.
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Old 01-11-2018, 10:27 PM
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Originally Posted by kiznarsh View Post
Just to close this out, the dealer diagnosed the problem to be the brake booster. They discovered the noise coming from there and decided to replace it after no luck with lubricating the pushrod/bellows assembly.

Part was expedited from the Motherland and I picked up the car today after driving a base Macan as a loaner for 5 days. For a CUV, it actually wasn't too bad. It's essentially the same instrument cluster so it was a familiar setting, but I felt like I could see the earth's curvature from being so high.
Seems odd the noise started after you did a brake fluid flush. Glad it's resolved now though. I had a Macan GTS as a dealer loaner, and a Cayenne also a few months later. If I'm going to drive a SUV, I want bigger rather than smaller. More comfortable ride, more room to ride and haul, and better capability to be a truck to do things like haul trailers, etc.
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Old 01-12-2018, 01:59 AM
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Originally Posted by okie981 View Post
Seems odd the noise started after you did a brake fluid flush. Glad it's resolved now though. I had a Macan GTS as a dealer loaner, and a Cayenne also a few months later. If I'm going to drive a SUV, I want bigger rather than smaller. More comfortable ride, more room to ride and haul, and better capability to be a truck to do things like haul trailers, etc.
Yeah still unclear how that happened...maybe defective booster? Naturally they had to bleed the brakes again so now my system is thoroughly fresh.

And what's up with everyone getting good loaners except me?

I agree about wanting something bigger though. The smaller SUVs (e.g. Q3) all have steeper rear windows, robbing you of even more space.
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Old 01-12-2018, 12:39 PM
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What brake fluid did you use to flush the brakes? This sounds a lot like the issue I had when I used ATE 200, except mine was a squeaking noise that sounded like it was coming from the booster but is was the master cylinder seals making the noise. The sound went away after flushing again with RBF600.
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Old 01-12-2018, 04:26 PM
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Lots of folks have issues after bleeding. Sometimes on older cars, it's pushing the piston further into the master cylinder than it has ever gone before, which can loosed rust or maybe tear a seal. And sometimes improper bleeding can get air in the system, which requires, yes, more bleeding. A 981CS shouldn't be old enough to have these kinds of issues, but I suppose that moisture could have gotten in there and caused some problems even in just a few years.

I was helping a friend bleed the brakes on his old P.O.S. Chrysler minivan, and I burst the rusted steel brake lines. I told him, better to find out in the driveway than at 80 MPH on the Interstate.
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Old 01-12-2018, 05:15 PM
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There was a month between bleeding and the booster issue surfacing, so they may not be related at all.

Regarding brake bleeding, I'll offer a few thoughts based on my experience. Your mileage may vary...
  • Flush and bleed per Porsche interval recommendation (you'd be amazed how much sediment will build up without doing so)
  • I'm not a fan of using the "pump the brakes" method (way more pressure than needed), rather prefer Motive Power Bleeder
  • Do the clutch slave at the same time (complete system flush), you'd be surprised how hot clutches get
  • Be really gentle with bleeder screws, most are threading into aluminum calipers... don't bugger up the threads, take your time
  • Use whatever brake fluid Porsche recommends unless you regularly track the car (fancy stuff is complete overkill in a street car)
  • In my experience, one does not need to bleed the ABS unit unless you open it up (which you should not), leave it alone
A standard Porsche braking system in proper working order is about as good as it gets. But they do require normal maintenance. Take care of them and you get some of the best braking in a road car available anywhere.

Good motoring!

Dave

'05 987S
'92 968 SP3
'88 944 NA (gone)
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Old 01-12-2018, 05:17 PM
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Originally Posted by DJ99 View Post
What brake fluid did you use to flush the brakes? This sounds a lot like the issue I had when I used ATE 200, except mine was a squeaking noise that sounded like it was coming from the booster but is was the master cylinder seals making the noise. The sound went away after flushing again with RBF600.
Yup, same one I used:



Weird that a different fluid made the noise go away. Is RBF600 more viscous?

Originally Posted by MJBird993 View Post
Lots of folks have issues after bleeding. Sometimes on older cars, it's pushing the piston further into the master cylinder than it has ever gone before, which can loosed rust or maybe tear a seal. And sometimes improper bleeding can get air in the system, which requires, yes, more bleeding. A 981CS shouldn't be old enough to have these kinds of issues, but I suppose that moisture could have gotten in there and caused some problems even in just a few years.

I was helping a friend bleed the brakes on his old P.O.S. Chrysler minivan, and I burst the rusted steel brake lines. I told him, better to find out in the driveway than at 80 MPH on the Interstate.
The pedal may have been pushed in more than it had been before, but the car's only 2 years old with 10k miles. But I doubt it was depressed any more than it would've if I was experiencing brake fade on the track, for example. Next time I'll probably just loosen the bleeder screw a little more and not touch the pedal just to be safe.
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Old 01-12-2018, 06:49 PM
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Originally Posted by kiznarsh View Post
Yup, same one I used:



Weird that a different fluid made the noise go away. Is RBF600 more viscous?.
Yea, RBF is thicker. It's weird because a lot of people use ATE200 without any problems. Must be a bad batch going around.
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Old 01-12-2018, 07:54 PM
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Originally Posted by Dave in Chicago View Post
There was a month between bleeding and the booster issue surfacing, so they may not be related at all.

Regarding brake bleeding, I'll offer a few thoughts based on my experience. Your mileage may vary...
  • Flush and bleed per Porsche interval recommendation (you'd be amazed how much sediment will build up without doing so)
  • I'm not a fan of using the "pump the brakes" method (way more pressure than needed), rather prefer Motive Power Bleeder
  • Do the clutch slave at the same time (complete system flush), you'd be surprised how hot clutches get
  • Be really gentle with bleeder screws, most are threading into aluminum calipers... don't bugger up the threads, take your time
  • Use whatever brake fluid Porsche recommends unless you regularly track the car (fancy stuff is complete overkill in a street car)
  • In my experience, one does not need to bleed the ABS unit unless you open it up (which you should not), leave it alone
A standard Porsche braking system in proper working order is about as good as it gets. But they do require normal maintenance. Take care of them and you get some of the best braking in a road car available anywhere.

Good motoring!

Dave

'05 987S
'92 968 SP3
'88 944 NA (gone)
All good points, thanks. And yes, I did bleed the slave cylinder. Fortunately my buddy had a crowfoot to make that possible, but it was with a 3/8" drive so bulkier than needed. I'm going to buy an 11mm flare nut crowfoot for a 1/4" drive to make life easier next time. Also didn't touch the ABS unit.

Oh...and to clarify, the problem surfaced essentially right afterwards. Once I got home after driving it for a couple of miles, I didn't drive it for a week and that's when I noticed it. I just didn't get around to taking it to the dealer until 1 month later.
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