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997.2 GT3 brake problem - long pedal when hot

mm450exc

Well-known member
Joined
13 Jun 2009
Messages
98
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?
 
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.
 
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?
 
mm450exc said:
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.
 
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.
 
:?: :?:

Have you tried heat paint on the disk & caliper to get a read on the temps?
 
I have not. A friend who has the same problem on a gen1 did.

He said the temperatures never go much over 300F.
 
If it's not heat then its air. Cant imagine what else.
 
Senoj said:
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.
 
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 :paperbag:

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