I have a couple of times in the past had an issue when braking late and hard at the track. The pedal goes rock hard, and the car simply stops slowing down. Its a very frightening experience. I can't feel the ABS through the pedal. I have heard others having similar experience. It only seems to happen when brakes are hot (after a few laps) and seems to be worse when tires are very worn. It has only happened to me on big braking zones (braking from 200+ KPH for 2nd or 3rd gear corner).

Recently I had my brake calipers changed to the GT3 6 pot calipers (similar to PCCB calipers but I have steel discs). This seems to have made the problem worse. 2 weeks ago I went off the end of the straight onto the grass probably 50% of the time when I braked late and hard. I either braked gently and early and had no issue, or braked aggressively and had no choice but to drive straight off the track (luckily for me the track in question has a run off road at the end of the straight). It wasn't simply a matter of mistiming the braking. I would brake 10 meters later and need an additional 50 meters or more to stop!

I had my fluid bled and replaced twice, and even the ABS system flushed. I bought new tires. These things helped, but the problem has not gone away completely. As a result on certain corners I have lost a lot of speed and confidence. I feel as though if I brake hard enough to let the ABS engage I will simply loose my brakes all together.

I suspect its an ABS issue. But then it would never cause an issue on the street (unless I was repeatedly braking from 200 to 0). The problem is becoming more pronounced the faster I drive and harder I brake. It never happened 12 months ago, but I think thats because I was driving more gently.

Has anyone else had this issue? Can anyone shed any light on what the problem might be? How about the more experienced guys racing their Caymans, Boxsters or 911's?

Any help would be greatly appreciated.

I am a novice driver, but I have had this happen with track pads...but not OEM pads.

One thought about ABS. If your pads and rotors are not stopping the rotation of your wheels enough to reach lockup, your ABS could be working just fine. But your car won't stop.

I never experienced significant fade with my OEM pads and people have speculated that my track pad experience had to do with the pads bedding in/ not getting them hot enough to work right. I moved from a 2.7 to a 3.4 and didn't get a chance to run the pads again.

When you say that it happens when your brakes are "hot", I wonder if it is actually the case that your pads are not running in their optimal temperature range. Maybe they are not hot enough?

I am sure the more experienced folks will have more to say.

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2007 Speed Yellow Cayman GT 2.7 (retired)
2008 Guards Red Cayman GT 3.4
"Carpe Cayman"

My pads were definitely hot enough. Probably too hot. It was a 35 degree (celsius) day, and as a result my discs are cracked to buggery and my once yellow front calipers are now orange! (Just from this one session). I can get the ABS to kick in if I hit the pedal hard no problem. These are Calipers that are the same as on the GT2, and I have high end track pads.

I suspect the ABS in the Cayman just can't deal with the situation, but perhaps it would work fine with factory pads and tyres?

I am not sure. It could be brake fade from the big stops - but it couldn't be the fault of the calipers, discs, pads because they don't make them any better than what I have. So all I can think is that it's the ABS or maybe Master Cylinder or other components?

P.S. Of this happens to anyone else, you can get the brakes working again by releasing completely and then hitting the pedal a second time. But obviously you loose a lot of ground while you do this so if you have braked late and have a wall in front of you it might not help.



DT

your pedal goes rock hard = loss of servo vacuum.

Read Hydraulic brake - Wikipedia, the free encyclopedia

under vacuum booster. Somehow you've lost vacuum.

See how that works: http://www.kfz-tech.de/Engl/Bremskraftverst.htm

stuck valve???

Crikey!!! Porsche brakes are supposed to be the BEST!! Are you using aftermarket pads? What kind? Are you using a racing brake fluid? What kind? More details, please. Obviously, the Porsche factory supported race cars do not have this problem, so the solution can be found by examining your specific brake system components. I suggest you contact Ernie at Mantis Sport (a site sponsor, and very successful Cayman racer).

Thanks. The loss of Vacuum is what it feels like. It seems to happen most often when I get on the pedal too hard at high speed, and then then keep braking for a long time after this. So it could be my aggressive braking technique. Maybe I have changed my technique without realising it now I am going faster, and perhaps the bigger brakes are making it worse. Still something does not seem right. I would expect if I brake a few meters too late to miss the apex and end up on the outside edge of the track, but not come flying off it!

I am using Castrol racing fluid and Pagid Orange Pads (I think!).

I agree with caySman that your problem seems like a loss of brake boost. However, that's usually consistent and not an intermittent problem as you are experiencing. Total speculation, but perhaps your brake system changes are exhausting the boost reserve. From personal experience, without brake boost a CS is very hard to stop.

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Dan

possibilities:

1) Your new multi piston brake calipers are too much for the master brake cylinder to handle consistently, thus under too much application the vacuum exhausted (unlikely)

2) Pressure leak in brake system, undetectable during normal street use, but comes up during extreme use as vacuum exhaustion. (most likely, as you say, if you brake long and hard suddenly you lose vacuum).

I think it's time to stop driving it now and flatbed it to the dealer for examination. They will probably bleed it and use flourescent dye to detect the leakage.

After a couple of incidents, I drove the car hard for another few sessions last time and it was OK. One of the guys who owns the company that installed the brakes (and was also our club champ last year) drove the car hard and had no problems. But he also had the same thing happen ONCE on his modified Base Cayman last year. So its a strange one. New tyres also seemed to help (its worse with worn tyres) which made us think it was a case of ABS not keeping up.

DT

I agree with Dan on what's happening

And, I agree with caySman here, too. My guess is the bigger brakes - 6 pots - have done a couple of things... first, there's not enough capacity in the system to handle them. Check the part numbers on the GT2 and GT3 cars that use these big calipers, and I'll bet you'll see a higher-capacity master cylinder and braking system. Second, I'll bet the bigger brakes up front have also messed with the braking bias and balance, and this probably needs to be re-thought and the bias re-done. Personally I think you should go back to the stock system, they may only have 4 pots, but they were engineered for the car in terms of bias and balance, and the the engineering spec for them is way, way more than the car needs... I'm a DE instructor and advanced level driver, and I've yet to out-drive my stock system with the regular 'ol OEM pads, using regular 'ol OEM brake fluid...

brad

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21-year PCA Member
PCA DE Instructor

The other possibility is that you are simply expecting too much. The Porsche brakes are great, but not magic.

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Life is not a rehearsal

Hi beez. Doesn't Electronic Brakeforce Distribution on the Bosch electronics negate the braking bias and balance issue? Yes natively the brake bias is screwed for sure, but doesn't EBD sort out the issue as far as the vacuum depletion issue is concerned?

I had this same problem a few times - exactly as you say, it was only after a few laps in the dry (although nowhere near as hot as 35 degrees!) and I was using Pagid RS29s in the front.

I concluded it was ABS malfunction, caused perhaps by unexpectedly more front brake bias (with the better pads). No noticeable ABS judder, but a rock solid pedal and no braking! It was a bit scary... I found if I took my foot completely off the brake and reapplied, then it seemed to work (assuming you didn't hit the barrier before then!)