Rob S

I’m looking for suggestions with this 944T brake problem.

At a recent track day (the first for me in this car, which belongs to a friend), the brakes faded badly. By "fading," I mean that the pedal was high and hard, but ineffective at slowing the car once the brakes got hot. They felt fine when they were cooler but after 5 or 6 laps they just didn’t generate the desired slowing no matter how hard the pedal was pushed. If I backed off for a lap or so, they regained their effectiveness until the pace was increased, and then they'd fade again. The pedal never got spongy, soft, or low. And I wasn't driving beyond 8 tenths.

The car is equipped with stock brakes *reportedly* fitted with Performance Friction 97 pads (now worn evenly to about the 35% level) and a fresh bleed with a good fluid (can't recall which). The rotors were nearly new in front, and look fine in the rear. And there’s are some front brake cooling ducts that were added by a shop. Those ducts have factory part numbers on them, but I don't know their origins.

Any idea what could cause this? It doesn't sound like anything hydraulic to me. If it were hot fluid, we'd lose the pedal. It sounds exactly like classic lining fade – where the pads are getting too hot and losing friction. But I ran identical brakes and pads on my trackable 944 S2 for years of very hard running with absolutely no such fade, ever.

A local shop suggested that it could be the master cylinder. He said that the aluminum master cylinders in that car are known to leak internally, and when they do so, it can cause a hard pedal. I’m having a hard time believing that a failed master cylinder could cause this sort of symptom. Can it? He also suggested it could be a leak of some sort related to the brake booster. But why only when the brakes are hot? And when the brakes were faded, stepping harder on the pedal didn't seem to help *at all* which is not consistent with a brake booster issue.

Normally, I’d declare this to be symptomatic of the wrong friction compound, and that a more race-like compound would solve it. But those *are* a race compound. Or at least I'm told they're PF 97s, but there’s no way for me to confirm by looking at them. And the next step up (the PF 01) is not available in that size. Could it be that the pads are so low on friction material (35%) that they’re overheating? I never had that happen with my car – I regularly ran them nearly all the way to the backing plate. I hate to suggest that the owner replace the pads wtih new PF 97s, because I’m afraid he may have the same problem again. And I hate to replace the M/C when it doesn’t seem logical to me that it would be the cause. But leaving as is won’t work either…

Got ideas? Any help would be appreciated. If you think it's the pads, what pads would you recommend?

Rob

67King

If it isn't the pad, it is probably the booster.

Rob S

Well, I don't know that it's not the pad. I still suspect the pads, but I can't understand why they'd act this way, given my experience with this pad in my own car. And if it were the booster, I would think that it would just cause a higher pedal effort, and that braking effectiveness would be proportional to pedal force. But that doesn't seem to be the case. The braking effectiveness just flattens out (at too low a level), and seems to be independent of pedal force.

It's intersting that there's never a brake burning smell during these episodes, though. The brakes just simply won't slow the car...

f1rocks

Have you pulled the pads out to see how glazed they are?

fasterfaster

What rotors are you using? Could be the combo of that pad and rotor, which would explain different performance from the pad than you've had in the past.

Rob S

The pads do not appear to be glazed at all, but the car was driven home from the track (about 80 miles) and then around town a bit, so I'm not sure if that might have cleaned off any of the glazing, had it ever been there. Rotors are stock on this car, which is what I ran on my car, and they're in perfect condition.

I can't explain it...

Van

PF97s are quite good, and can withstand a lot of heat. I'd suspect those aren't the pad that's in there.

Sounds like stock pads to me.

Eric_Oz_S2

PFC97s have somewhat unique characteristics. If they generate a LOT of dust AND the dust is really hard to remove (it is quite an abrasive compound) then they could be PFC97s. The PFC97 dust sets like concrete when it gets wet and dries again. Also PFC97s squeel a lot when cold on the street. If you ran PFC97s on your S2 previously you should know by how they grab, dust and sound if they are the same pad as you used previously.

I have never had a problem with mine even under extreme track use on standard rotors.

Tedro951

Plenty of threads on pads, but it takes less time to pull the pads than it does to read all of this...

Rob S

I'm glad to get your perspective, and that of all who have responded. I do believe that these are likely to be PF 97 pads. And with all the good service I had from them in the past, I'm perplexed as to why they seem not to be performing on this car.

So I'm beginning to reconsider the possibility that there's another thing going on here and that it may involve the booster and possible vacuum issues, as suggested by 67 King. It's been brought to my attention that the vacuum system on 944Ts is relatively complex and occasionally problematic, At first, I dismissed the vacuum issue, because my sense was that during these episodes of "faded" brakes, the pedal force was not proportional to the decel I was getting. But I may have been wrong. Maybe, without enough vacuum for the brake booster, the pedal force goes so high that it makes it feel like faded brakes. But why only when the car is hot? Could it be that on those "hot" laps, I'm on the turbo boost longer, giving less opportuntiy for the brake booster to be replenished with vacuum? And that maybe there's a leak somewhere so that whatever vacuum is accumulated during closed- or part-throttle is lost to the leak? Maybe the brakes "recover" when I back off because there's just enough more closed-throttle time to replenish the brake vacuum booster and give me some assistance in braking.

If it were a vacuum problem, does anyone have any suggestions on where to look first, or how to confirm that this is what is happening? Ideally, I'd have a gauge on the pressure (or vacuum) inside the brake booster. But that may not be easy to do. Is there a check valve in that system that may have failed -- one that's designed to preserve vacuum in the brake booster and prevent turbocharger boost from intruding? Or is there some other common failure mode I should be looking for? I've noted there are many threads about vacuum manifolds on the 944T and how people have bypassed them. Any suggestions for getting to the bottom of this quicky?

67King

Rob, I've had two friends lose vacuum to the brake booster at the track. Each one of them was adamant that there was a bigger problem and that the car "would not slow down, no matter how hard I pushed!" I think that the track environment is just so much more demanding than the street that the proportionality of stopping force to applied pedal pressure is not as easy to percieve. Each car was relatively heavy (~3000 lbs), one was a 944T.

But one thing you can check at the track is for glazing. You can always scrub them against concrete, but I now carry sandpaper with me, I think 80 grit, in caes I need to scrub down pads.

Van

Statement of the month!!

This may help:

Rob S

67 King, thanks for that report. Maybe we really do have a vacuum issue that's masquerading as a brake fade issue. If it were truly fade, I wouild think that we should see it in the pads (and maybe rotors) and likley smell it while driving -- neither of which is the case. The pads look just fine.

Van, thanks for the video. We may have to try something like that. Wish I lived next door so I could borrow that nifty yellow cap/adapter you created for vacuum testing.

A decision has been made. We're going to replace the pads with new PF 97s, bleed the system with good fluid, and try it again. From all reports, including a good discussion with Derrick at Peformance Friction, it seems the 97 is a perfectly adequate pad and that it should not be necessary to go to anything different. If the problem recurs, we'll dig into the vacuum system.

If it were a vacuum problem, though, does anyone have in mind a likely culprit? A problem could manifest as 1) too little vacuum being fed to the booster, perhaps because of a cracked or leaking feed line, or 2) vacuum escaping from the booster somehow. I keep thinking that there must be some sort of check valve in the brake booster line that may have failed. Is there, and if so, is it known to fail?

Van

There is a check valve inline to the booster hose, and it can fail. Under boost, if the check valve is dead, there won't be any more vacuum in the brake booster. Something like that will pass my pressure test, too...

Upstate101

Sounds a lot like my 944 turbo brakes. Its a 1986 with stock rotors and new Pagid Sport pads. New SS lines. After sensing far more needed pedal effort than in my E36 M3 during HPDEs, I had the master cylinder , booster, and check valve all replaced and replaced fluid with Castrol SRF. Brakes are better but still require significantly more pedal effort than M3. I think I'll try more aggressive race pads next and see if a higher friction pad will result in less needed pedal effort for threshold braking. Not trying to hijack OPs thread - just seems like a similar problem.