Some good suggestions here.

These are floating rotors.

Good question. I'll have to check with my shop.

I did use OEM front rotors before switching to the Stop Tech rotors and never had this problem with those. I only noticed the problem on the second DE event I did with the Stop Tech's. It's possible it may have been there on the first event after the switch and I missed it because I was still getting used to the overall feel of the new rotors.

The master cylinder was replaced after this problem started. It did improve the overall feel of the brakes but didn't eliminate the problem. It's always possible the new master cylinder was bad, but it seems like a long shot.

I will try that at the next event in a couple weeks.

 

This...Even my cup car has to be bled before any running on a new day. Just becomes part of the habit...that and Id run motul as well.

 

Interesting to see if it actually is pad knockback! Worn wheel bearings and other loose components should exacerbate this so, if tapping before the braking point fixes it, then it might be a good idea to check those as well?

(Not a regular poster here, been perusing the forum as I am considering a GT3 ).

 

I'd suggest going back to stock rotors again. If they work fine, that's your problem. Even though someone else may have success with ST rotors, your driving style is different, pads may be different and current rotor thickness may be different. The GT3 needs a wide annulus disc with a lot of heat capacity. If the ST discs are substantially lighter than stock, they may not have the heat capacity to be able to handle the stress of high friction pads on the track.

 

I have 996 GT3 350mm front brakes w/ PCCB MC on my Cayman S and also suffer a soft pedal w/ too much travel after a few laps. I've run SRF and now PFC fluid; OEM, Cayenne, and now Deman rotors; OEM, Hawk DTC70, and PFC 08 pads; stainless lines - no difference. My Deman rotors are looking rather cooked after 4000 miles and 4 DEs. I'm thinking that I have a heat issue. Is it possible that the pistons are not retracting, rubbing on the rotors and not allowing them to cool down? The calipers are 8 years old and have 32000 miles and 21 DEs on them. Were do we get rebuild kits?

 

First thing that comes to mind is lack of cooling (are you using any brake cooling ducts, i.e. .2 rear ducts). As a result, next is brake fluid (if using Motul or SRF then this is not it), followed by pads (did you bed them in properly or maybe wrong compound for your driving characteristics or are you over braking, in other words "cooking" your brakes), rotors (are they too thin or lack of cooling) and then calipers (use/time or lack of cooling has made certain parts fail).

I don't play golf, but I understand the concepts of it. Brakes are like golf clubs...different swings require different clubs, different holes require different clubs. You shouldn't use a driver to putt, just as you shouldn't use Pagid RS14 (blacks) to run an endurance race. You could, but shouldn't.

Not all brakes work for all tracks not even at the F1 level, just ask Hamilton in Canada or Verne yesterday. Different driving style makes a huge difference, just ask Rosberg in Canada.

One question, can you tell if you're losing any braking capacity up-front or in the rear?

Lastly, I don't think it's your MC at all...

 

Try bleeding the clutch slave & when you have your caliper bleeders open, tap (gently) the caliper side with a rubber mallet to dislodge any trapped air bubbles.
finally, I hope your caliper piston seals are in good shape.
GL!

 

+1 on SRF. ...

 

This is a perennial thread so I hesitate to suggest a different answer, but I have experienced the same problem on two different cars (both GT3s) and what has (at least for right now) solved it is a brake flush at the dealership, using the PIWIS tool to bleed the ABS module. It appears that the Durametric is not effective, regardless of what it says. For the first time since my system was opened in the spring, and after countless bleed cycles, I have a decent and consistent pedal. Unfortunately, no one but a dealer can afford the tool. I just hope this post doesn't jinx me next time out!

 

Got any vids of when u go thru braking zones? Any incar data systems? Mike

 

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.

 

The engineer by training in me says you should go back to stock and see if that eliminates it, then add components until problem arises

You have too many variables to eliminate right now

At least get rid of floating rotor and see if that helps

 

Thanks for the head's up on the Durametric.

 

Agree too many variables to play with, need to eliminate the one that seems to be the issue, only commonality so far is your rotors, new pads, new fluid, and doing the brake check on track. Last data point would be if old rotors back on and no issues, then you have a strong case for knock back.

 

Isn't the pedal longer with floating rotors? U literally have to be aggressive on them. Not just a progressive jab of pedal but immediate. Always get a kick hearing them clank at pregrid. Mike