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mm450exc
Trainee


Joined: 13 Jun 2009
Posts: 80



PostPosted: Wed Aug 19, 2015 6:51 pm    Post subject: 997.2 GT3 brake problem - long pedal when hot Reply with quote

So I've been messing around with this forever. After a few laps on track pedal gets long. If you pump once pedal is 100%.

Here is the setup:

Alcon with RS29 pads
ATEC brake lines
New Master Cylinder
New ABS Module (One circuit did not cycle on the faulty unit)
SRF brake fluid

It's like knockback but only when hot. Any ideas?
 
  
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Senoj
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Joined: 23 Jan 2006
Posts: 5024
Location: Sussex


PostPosted: Wed Aug 19, 2015 7:14 pm    Post subject: Reply with quote

assuming air is purged it has to be too much heat, cant really see how with that spec unless pads are worn, in the last half of their wear maybe ? Potentially getting hotter than youd want.

i have pretty much that exact same set up and will get a very small anount of "long" pedal after a few laps with worn ish pads, its noticeable but car still stops well enough.
 
  
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spyderman
Suzuka


Joined: 26 Jul 2009
Posts: 1094
Location: near Milton Keynes


PostPosted: Wed Aug 19, 2015 7:35 pm    Post subject: Reply with quote

Not bled properly.
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NXI20
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Joined: 02 Feb 2008
Posts: 3234
Location: South Bucks

2004 Porsche 996 GT3 Mk2

PostPosted: Wed Aug 19, 2015 7:42 pm    Post subject: Reply with quote

With that list of bits, I'd suggest that it's perhaps down to your brake technique. I've done 40+ minute sessions at Spa in my 997.2 GT3 on standard brakes & RS29 pads without issue.

What is your brake technique & what circuit were you at when you experienced this?
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Need Alcon discs or Pagid pads? https://uber9s.com

2004 GT3 CS in Atlas Grey with too many mods to list!
1995 993 GT2 recreation in Polar Silver
2010 GT3 CS in Riviera Blue Smile
1978 Carrera SC Barn Find in Red (restoration project)
 
  
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DynoMike
Barcelona


Joined: 25 May 2012
Posts: 1285
Location: The Cotswolds

2003 Porsche 996 Turbo

PostPosted: Wed Aug 19, 2015 7:46 pm    Post subject: Re: 997.2 GT3 brake problem - long pedal when hot Reply with quote

mm450exc wrote:
So I've been messing around with this forever. After a few laps on track pedal gets long. If you pump once pedal is 100%.

Here is the setup:

Alcon with RS29 pads
ATEC brake lines
New Master Cylinder
New ABS Module (One circuit did not cycle on the faulty unit)
SRF brake fluid

It's like knockback but only when hot. Any ideas?



We had this when running a bike racing team. We changed everything, pads, discs, lines, fluid and master cylinder. It was cured by fitting new calipers at the meeting.

The culprit was overheated seals which then left the pad in contact with the discs. After 3 laps of a short circuit, the lever would come back to the bars. We renewed the seals in the original calipers and used them again on one of the spare bikes, no issues with braking at all.

We traced the initial cause of the problem to be over use of chemical brake cleaners, which hardened the seals, which in turn meant that the seals did not claw the pistons back when letting go of the brakes. This then cooked the seals alive and SRF boiled..... From this time on, the bikes were prepped with warm soapy water and bottle brushes on all of the calipers, not aerosols.

It might be worth having a look at the seals.
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mm450exc
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Joined: 13 Jun 2009
Posts: 80



PostPosted: Wed Aug 19, 2015 7:49 pm    Post subject: Reply with quote

Nick - it's Mike.

Silverstone and SPA. Brake maybe not as aggressive as you. Can't be down to braking as brakes look good - so not overheating.
 
  
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NXI20
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Joined: 02 Feb 2008
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Location: South Bucks

2004 Porsche 996 GT3 Mk2

PostPosted: Wed Aug 19, 2015 7:53 pm    Post subject: Reply with quote

Question Question

Have you tried heat paint on the disk & caliper to get a read on the temps?
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Nick
Need Alcon discs or Pagid pads? https://uber9s.com

2004 GT3 CS in Atlas Grey with too many mods to list!
1995 993 GT2 recreation in Polar Silver
2010 GT3 CS in Riviera Blue Smile
1978 Carrera SC Barn Find in Red (restoration project)
 
  
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mm450exc
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PostPosted: Wed Aug 19, 2015 8:06 pm    Post subject: Reply with quote

I have not. A friend who has the same problem on a gen1 did.

He said the temperatures never go much over 300F.
 
  
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Senoj
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Joined: 23 Jan 2006
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Location: Sussex


PostPosted: Wed Aug 19, 2015 8:13 pm    Post subject: Reply with quote

If it's not heat then its air. Cant imagine what else.
 
  
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NXI20
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Joined: 02 Feb 2008
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Location: South Bucks

2004 Porsche 996 GT3 Mk2

PostPosted: Wed Aug 19, 2015 9:13 pm    Post subject: Reply with quote

Senoj wrote:
If it's not heat then its air. Cant imagine what else.


+1

Assuming that standing on the pedal for a minute when cold doesn't make it sink slowly it really can't be anything else.
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Nick
Need Alcon discs or Pagid pads? https://uber9s.com

2004 GT3 CS in Atlas Grey with too many mods to list!
1995 993 GT2 recreation in Polar Silver
2010 GT3 CS in Riviera Blue Smile
1978 Carrera SC Barn Find in Red (restoration project)
 
  
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Disco
Estoril


Joined: 13 May 2008
Posts: 3637
Location: Hertfordshire

2010 Porsche 997 GT3

PostPosted: Wed Aug 19, 2015 10:45 pm    Post subject: Reply with quote

Mine (also a 997.2 GT3 on Alcons & RS29s) does that when the pads are close to spent. Once you are used to it, it is as good an indication of pad wear as sticking your head in the wheel well with a torch. If it goes longer every time you hit the pedal, you are into the backing plates Paper Bag

[Yep - years back when I used to have my 996 I did really do that. New disks were called for]
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Cunno
Watkins Glen


Joined: 13 Dec 2008
Posts: 2174
Location: Nottinghamshire


PostPosted: Thu Aug 20, 2015 12:10 pm    Post subject: Reply with quote

Are you sure it's fade? The reason I ask is that the 997 abs system is different to 996s. You can get the abs system to cut in and trigger what people refer to as the ice mode. Basically it feels like fade, peddle goes soft and you seem to have about 60% or less of your normal braking force, but it's the abs system going nuts. Can be triggered in 2 ways bumpy track surfaces within braking zone and going to aggressive from throttle to brakes.

When it happens if you come off the brake and re apply does it cure itself ? If so this is the abs system and you need to modify your braking style or buy a cup abs system at £8000 Sad

If every thing else looks ok I expect this is what's happening.

Some back ground here.

http://rennlist.com/forums/997-gt2-gt3-forum/534934-well-abs-ice-mode-finally-got-me.html

http://rennlist.com/forums/997-gt2-gt3-forum/866677-abs-delete-and-ice-mode.html


http://rennlist.com/forums/997-gt2-gt3-forum/801844-ice-mode.html
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NXI20
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Joined: 02 Feb 2008
Posts: 3234
Location: South Bucks

2004 Porsche 996 GT3 Mk2

PostPosted: Thu Aug 20, 2015 12:17 pm    Post subject: Reply with quote

Cunno wrote:
When it happens if you come off the brake and re apply does it cure itself ? If so this is the abs system and you need to modify your braking style or buy a cup abs system at £8000 Sad


Nope, the Cup system isn't the answer either as you can't re-program it. You need the £12K Bosch ABS unit that you can program. Then you need to employ a guru to program it. Call it £20K.
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Need Alcon discs or Pagid pads? https://uber9s.com

2004 GT3 CS in Atlas Grey with too many mods to list!
1995 993 GT2 recreation in Polar Silver
2010 GT3 CS in Riviera Blue Smile
1978 Carrera SC Barn Find in Red (restoration project)
 
  
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mm450exc
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Joined: 13 Jun 2009
Posts: 80



PostPosted: Thu Aug 20, 2015 12:28 pm    Post subject: Reply with quote

It's not fade. Got brakes only longer travel on the first press. Yes - re-apply and brakes are 100% (rock hard as it should be!).

I stay out longer. I already brake late but will make an effort to brake even shorter/harder/less and see what happens.

Will take the brakes apart and see if I find anything.

It's not knock-back. But did anybody install anti-knockback springs? Could that help?
 
  
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mm450exc
Trainee


Joined: 13 Jun 2009
Posts: 80



PostPosted: Sat Aug 22, 2015 4:04 pm    Post subject: Reply with quote

Problem found! Very much uneven pad wear. Pad 10mm at top and 5mm at bottom! So was basically running on 50% of the pad - heat!
 



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Senoj
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Joined: 23 Jan 2006
Posts: 5024
Location: Sussex


PostPosted: Sat Aug 22, 2015 8:01 pm    Post subject: Reply with quote

Heat would imply fade, but you said no fade?

Those of us that are economically minded would flip the pads left to right after about 40% wear, this way you can get max use out of them. RS29's do seems to fall off towards the end of their thickness. One of the good things about PF is that you can run them to the backing plates pretty much, endless too. I assume this is to do with the back plate material?
 
  
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mm450exc
Trainee


Joined: 13 Jun 2009
Posts: 80



PostPosted: Sat Aug 22, 2015 8:30 pm    Post subject: Reply with quote

Will flip them going forward. And will be using RS19 next.
 
  
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Cunno
Watkins Glen


Joined: 13 Dec 2008
Posts: 2174
Location: Nottinghamshire


PostPosted: Sat Aug 22, 2015 9:21 pm    Post subject: Reply with quote

I had a similar uneven wear on r29 pads, what are we saying is the course?
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Senoj
Zolder


Joined: 23 Jan 2006
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Location: Sussex


PostPosted: Sat Aug 22, 2015 9:51 pm    Post subject: Reply with quote

Any pad wil wear like that. The bottom of the caliper grips first and hardest, the "front" of the grip so to speak. The pistons are different sizes to compensate and even out the pressure but its still that part of the pad that hits the disc first.
 
  
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mm450exc
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Posts: 80



PostPosted: Sat Aug 22, 2015 10:53 pm    Post subject: Reply with quote

Different calipers then?

A friend has the Alcon kit which comes with Alcon calipers on a GTR which is a lot heavier than a GT3. No problems at all.
 
  
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