Why does my brake pedal go soft on the track?
06-27-2014, 10:28 AM
#31
Rennlist Member
What you initially described is knock-back.
If you go into a brake zone, tap the pedal first to firm it up, then brake hard....if your pedal stays firm, then it's likely just knockback, which can be addressed.
If you tap to firm it up, and then your pedal still gets soft while braking, then it's something else, or a combination of both.
I honestly used to get lots of knockback with stoptech's on my M3, but it's also because when running -4 degrees, the front wheel bearings get torched.
To cure knockback, either go to 2-piece rotors, such as AP's (or 1 piece), or tap the pedal prior to braking. As I said before, I noticed, albeit very minimal, some knockback when switching from AP to PFC. So you give up some ultimate firmness on application for less overall drag on the system. It also gives the car more rear initial bias on initial application, if you don't tap the pedal first in brake zones where you consistently have knock-back. Which again, can be good or bad depending on how the car is setup. Mine is setup pretty free, so it caused even more rotation which I had to address.
If you go into a brake zone, tap the pedal first to firm it up, then brake hard....if your pedal stays firm, then it's likely just knockback, which can be addressed.
If you tap to firm it up, and then your pedal still gets soft while braking, then it's something else, or a combination of both.
I honestly used to get lots of knockback with stoptech's on my M3, but it's also because when running -4 degrees, the front wheel bearings get torched.
To cure knockback, either go to 2-piece rotors, such as AP's (or 1 piece), or tap the pedal prior to braking. As I said before, I noticed, albeit very minimal, some knockback when switching from AP to PFC. So you give up some ultimate firmness on application for less overall drag on the system. It also gives the car more rear initial bias on initial application, if you don't tap the pedal first in brake zones where you consistently have knock-back. Which again, can be good or bad depending on how the car is setup. Mine is setup pretty free, so it caused even more rotation which I had to address.
06-27-2014, 11:04 AM
#32
Rennlist Member
FWIW, I haven't noticed any knock back with Brembo Type 3 floaters.
06-27-2014, 01:38 PM
#33
Drifting
FWIW: I make/ design 2 pc fronts and rear rotors -- no knock back on any of them- that is why i really do not think this is the case. If you think it might be--- check by going back to oem rotors-- one of your rotor hats may not be straight.
06-27-2014, 02:36 PM
#34
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I'm just back from three days at the track (two at LRP and one at the new Thompson Speedway in CT) and here's what I've learned on the track.
First, before going to the track, I had my shop flush the brake fluid and refilled with Motul RBF660. They routinely recycle the ABS pump as part of the process. However, I don't think they use the PIWIS tool that someone mentioned. Then I had new PFC08's installed on the front because the old pads were worn below the 50% allowable limit for PCA DE. The car left the shop with a very firm brake pedal on the street.
I had temperature strips mounted on all four calipers and they indicate that the calipers don't even reach 300F on the track. That was surprising. I know the rotor temperatures go much higher because I've measured 350F in the fronts with an IR pyrometer and that's after a cool-down lap, so they must be much hotter on the track. Therefore I doubt that the brake fluid is even close to its boiling point.
Tapping the brake pedal lightly before hard braking firms up the pedal every time. But I find this gets annoying and upsets my braking rhythm. I'm just not used to that routine.
Wheel bearings show absolutely no play and are not at all noisy, so I tend to think it's not a problem with worn wheel bearings causing pad knockback. I'm not ruling out the knockback theory, which may be plausible, since I'm using two-piece floating rotors (Stoptech slotted) that are said to be prone to the knockback effect. However, I know other people using these same rotors (in a 911 but not a GT3) and they don't experience knockback.
Someone suggested going back to the OEM drilled rotors. I stopped using the drilled rotors because their lifetimes on the track are short, so I went to slotted which last much, much longer. The cracks around the holes on the drilled rotors is what kills them.
So I'm not sure I have the answer yet, but there are still a couple theories in play. More ideas are welcome here.
First, before going to the track, I had my shop flush the brake fluid and refilled with Motul RBF660. They routinely recycle the ABS pump as part of the process. However, I don't think they use the PIWIS tool that someone mentioned. Then I had new PFC08's installed on the front because the old pads were worn below the 50% allowable limit for PCA DE. The car left the shop with a very firm brake pedal on the street.
I had temperature strips mounted on all four calipers and they indicate that the calipers don't even reach 300F on the track. That was surprising. I know the rotor temperatures go much higher because I've measured 350F in the fronts with an IR pyrometer and that's after a cool-down lap, so they must be much hotter on the track. Therefore I doubt that the brake fluid is even close to its boiling point.
Tapping the brake pedal lightly before hard braking firms up the pedal every time. But I find this gets annoying and upsets my braking rhythm. I'm just not used to that routine.
Wheel bearings show absolutely no play and are not at all noisy, so I tend to think it's not a problem with worn wheel bearings causing pad knockback. I'm not ruling out the knockback theory, which may be plausible, since I'm using two-piece floating rotors (Stoptech slotted) that are said to be prone to the knockback effect. However, I know other people using these same rotors (in a 911 but not a GT3) and they don't experience knockback.
Someone suggested going back to the OEM drilled rotors. I stopped using the drilled rotors because their lifetimes on the track are short, so I went to slotted which last much, much longer. The cracks around the holes on the drilled rotors is what kills them.
So I'm not sure I have the answer yet, but there are still a couple theories in play. More ideas are welcome here.
06-27-2014, 02:43 PM
#35
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Is the pedal softer even when the car is stationary right after pulling in from track?
07-23-2014, 03:20 PM
#36
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As the OP of this thread, I'm reviving it because I continue to be frustrated and vexed by this problem. To refresh, what happens is that at the track, the brake pedal starts to sink after a few minutes. The brakes don't start to bite until the pedal brake pedal is about even with the throttle pedal and the feel is soft. Normally they bite much higher up and they feel firmer. Once this happens, the brakes never recover, they stay soft after things cool off, even on the street. (I think I misspoke earlier when I said the brakes firm up overnight after cooling off. They do not.) The car still stops fine but the pedal feel is poor.
Here is what I've tried:
- Flushed and bled the brake fluid many, many times. This has been done before every track event. We never, ever saw air bubbles coming out when doing a bleed. The brake pedal returns to firm after doing this. I'm currently using Stoptech STR-660 fluid and also tried Motul RBF660. Neither makes any difference. Also tried Castrol but the brake pedal was lousy from the start.
- We had the Porsche dealer do the flush/bleed with their PIWIS tool to activate the ABS pump on the theory that the Durametric tool doesn't do it right. No difference.
- Brake caliper piston seals and dust boots were replaced on all 4 calipers. Afterwards we checked regularly for free movement of the pistons and never saw any issues.
- Replaced the master cylinder. No difference.
- Changed front brake pads from worn PFC08 (worn below 50% PCA DE allowable limit) to new PFC08. No difference.
- Put temperature strips on all 4 calipers to see how hot things were getting. They never registered above 300F, which means it is highly unlikely the brake fluid is getting anywhere near its boiling point.
- One thing we noticed is that the calipers do flex noticeably under braking pressure. You can see this with the car just sitting and someone putting pressure on the brake pedal. We've observed this on other GT3's so I don't think my car is unique here.
What does firm up the pedal is a quick, light tap just before hard braking. That works every time. The tap is often accompanied by a light clanking noise from the front. It's been suggested that this is a characteristic of brake pad knockback from the 2-piece floating rotors in front.
We've checked for worn wheel bearings but they have absolutely no play and don't make noise like failed bearings.
For those of you who have dealt with pad knockback, is your experience the same as mine? Do you get this on the street, too?
I'm still looking for ideas here...
Here is what I've tried:
- Flushed and bled the brake fluid many, many times. This has been done before every track event. We never, ever saw air bubbles coming out when doing a bleed. The brake pedal returns to firm after doing this. I'm currently using Stoptech STR-660 fluid and also tried Motul RBF660. Neither makes any difference. Also tried Castrol but the brake pedal was lousy from the start.
- We had the Porsche dealer do the flush/bleed with their PIWIS tool to activate the ABS pump on the theory that the Durametric tool doesn't do it right. No difference.
- Brake caliper piston seals and dust boots were replaced on all 4 calipers. Afterwards we checked regularly for free movement of the pistons and never saw any issues.
- Replaced the master cylinder. No difference.
- Changed front brake pads from worn PFC08 (worn below 50% PCA DE allowable limit) to new PFC08. No difference.
- Put temperature strips on all 4 calipers to see how hot things were getting. They never registered above 300F, which means it is highly unlikely the brake fluid is getting anywhere near its boiling point.
- One thing we noticed is that the calipers do flex noticeably under braking pressure. You can see this with the car just sitting and someone putting pressure on the brake pedal. We've observed this on other GT3's so I don't think my car is unique here.
What does firm up the pedal is a quick, light tap just before hard braking. That works every time. The tap is often accompanied by a light clanking noise from the front. It's been suggested that this is a characteristic of brake pad knockback from the 2-piece floating rotors in front.
We've checked for worn wheel bearings but they have absolutely no play and don't make noise like failed bearings.
For those of you who have dealt with pad knockback, is your experience the same as mine? Do you get this on the street, too?
I'm still looking for ideas here...
07-23-2014, 07:09 PM
#37
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What you're describing is definitely a symptom of the pistons retracting too far. The fact that a light tap brings the pedal up means that the pistons are coming toward the pad and do not have time to retract before you hit them again. This results in less pedal travel after the tap. The clank you hear is from the pads rattling in the space created by the retracted pistons.
In my experience, this has always been from inadequate lubrication of the piston seals. You said that you had yours changed, and that the pistons move freely, so maybe it is indeed something else. If you still have the stock rotors, perhaps you could try those for a day and see if that makes a difference.
The only other thing I can think of is to replace the retaining spring on the front caliper.
Jon
In my experience, this has always been from inadequate lubrication of the piston seals. You said that you had yours changed, and that the pistons move freely, so maybe it is indeed something else. If you still have the stock rotors, perhaps you could try those for a day and see if that makes a difference.
The only other thing I can think of is to replace the retaining spring on the front caliper.
Jon
07-24-2014, 07:49 PM
#38
Instructor
As the OP of this thread, I'm reviving it because I continue to be frustrated and vexed by this problem. To refresh, what happens is that at the track, the brake pedal starts to sink after a few minutes. The brakes don't start to bite until the pedal brake pedal is about even with the throttle pedal and the feel is soft. Normally they bite much higher up and they feel firmer. Once this happens, the brakes never recover, they stay soft after things cool off, even on the street. (I think I misspoke earlier when I said the brakes firm up overnight after cooling off. They do not.) The car still stops fine but the pedal feel is poor.
For those of you who have dealt with pad knockback, is your experience the same as mine? Do you get this on the street, too?
I'm still looking for ideas here...
For those of you who have dealt with pad knockback, is your experience the same as mine? Do you get this on the street, too?
I'm still looking for ideas here...
After all the effort and expense, I'll still say, try another set of discs. Buy or borrow a set of OE discs just to test the theory that it may be the ST discs. With all the things you have tried, you haven't eliminated the discs as the potential issue.
03-25-2015, 01:13 PM
#39
You ever found the problem?
I got the same on my 997.2 GT3. Alcons all around with RS29 pads. ATEC brake lines and SRF brake fluid. Will get the master cylinder replaced next week and see.
I got the same on my 997.2 GT3. Alcons all around with RS29 pads. ATEC brake lines and SRF brake fluid. Will get the master cylinder replaced next week and see.
03-26-2015, 07:49 PM
#40
RL Community Team
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This really sounds like knockback esp if you have gone through the usual list of suspects.
The other possibilities would be one of the brake lines is getting soft.
Ray
The other possibilities would be one of the brake lines is getting soft.
Ray
04-10-2015, 07:13 PM
#41
Former Vendor
Remove the caliper (keep brake line ON) and give a reaaally good look-see at the pistons. Play with em. In/out - not too far out or it's a big mess. If it doesn't feel right, it aint't right.
