I experience a really strange brake problem this weekend at the DE. My pedal got soft, but only in one turn. It was strange, it only happened at a specific braking area which is a long way from the previous braking area (which is the hardest on the track) followed by some downhill twisties then uphill and finally a left hand off camber downhill sweeper. At the end of the strait, it felt like my pedal went almost to the floor, felt soft, but still worked. It happened every time.

At first I thought I was overheating the brake fluid (Castrol SRF which I haven't flushed/bleed in a while) but then why was it only happening in one area? One suggestion I got was that it wasn't the fluid, but could be the wheel bearing as the wheel wobbles and spreads the pads apart further than normal, requiring more pedal travel to get them to bite. That explanation worked for me until I talked to one of the guys that went off in that area. He said he was also using SRF and his pedal went to the ground. He flushed that week. That got me to thinking it was something else. Then it get's even weirder, I started hearing of others having the same problem in the same area. I'm stumped. I have no idea what it could have been. Now the weather could have something to do with it, it was in the mid to high 90’s and the humidity was above 90%.

Has anyone ever experience anything like this? Just too strange.

I posted this to the 993 forum, but thought I might get more answers here.

TIA,
George

 

Yeah, I'd be going for caliper piston knockback. Particularly if the brakes are fine for subsequent corners. I have a similar problem if I track out hard on the gators' teeth going onto the back straight at Waterford - no pedal on first apply. Another dead giveaway of knockback is that a second brake apply (pumping the brakes) give normal braking.

 

Vaughan,

I pumped the brakes to get the pressure back to normal. Sounds exactly like what you are talking about. Is that caused by a bad wheel bearing or something else?

Now for the other question, why were others getting the same problem in the same spot?

George

 

Probably brake pad knockback. See my post on the 993 board to your issue.

 

I remember many years ago at the glen being warned to check my brakes before entering turn 5 at the end of the long backstraight. this was before the bus stop was put in. In a stock car ike mine, you brake hard into the downhill turn 1, then don't use the brakes for a long time. This gives the fluid in the calipers time to boil without the brakes being applied to drive the fluid back into the caliper. Indeed, later the first day of the event, as I got faster and the temps got higher, I reached over with my left foot and tapped the brake on the way up the straight, and the pedal went to the floor. This was the only place it happened.

The dynamic is similar to your description: Lots of hard brake use, then a relatively long period of not using them, along with less-than-fresh fluid.

While it could be the wobbly wheel bearing thing, i think that other symptoms would manifest themselves if the play was that severe, and also it should not pass a PCA tech inspection with wheel bearing play. Change your fluid and pump up your brakes as you approach the spot where the problem occurs. And by all means confirm that your bearings are good.

One last thing, if the brakes aren't pulling on application, that suggests that for it to be bearings, both would have to be loose an exactly equal amount at the same time. This seems unlikely unless they were adjusted wrong.

 

I have never had this problem, but the answers on piston/pad knockback, kind of intrigue me. I have never heard of it, and I am not sure my guessing is even close to right. I looked on the 993 list and couldn't find SundayDriver's explaination. Somebody please enlighten me.

Bill Seifert

1983 944 Race Car

 

I have had exactly the same problem with the 993 I am currently racing. I changed all the wheel bearings and no change. I also replaced a bunch of brake parts and also no change. I am pretty stumped.

 

Make sure that the large nut on the rear axle is tight (300+ lb-ft). If the nut backs off, you will knock back the pads on the wheel where nut is loose.

BTDT (on the back straight at the Glen, incidentally)...

gb

 

I'll just repeat some of the stuff here.

First, bearings do not have to be very loose to cause this. I have had it happen and there is no pull or other obvious symptoms. But that is not the usual cause of knockback.

When you corner, you will get defelction in the rotor and bending of the caliper. If you generate enough forces and the parts are flexible enough, then the relative motion between the rotor and caliper pushes the pads back. Next time you apply the brakes, you use most (all?) of the pedal travel simply retruning the pads to the rotor. No clamping force until they return. Hence you get a long pedal, but the next application the brakes are fine. That does not happen with boiled fluid. In that case, there is vapor in the system and it continues to compress and the brakes never come back all the way.

Knockback is very common in formula cars and sports racers where you have very light parts and very high g's. Additionally, because the cars are so low, the master cylinder may be lower than the caliper so the fluid wants to flow out of the caliper.

Since this is driven by cornering forces and certain vibrations (driving over gator teeth), it will tend to repeat in certain parts of the track, but not others. Solutions:

Anti-knockback springs - These go behind the pistons and force the pads against the rotors.

Residual pressure valve - Goes in-line to maintain a small amount of pressure in the line to resist the knockback. Wilwood makes a 2 psi version for disc brakes. (I have no idea if this causes a problem with ABS).

Floating rotors - the rotor is allowed to float and therefore does not transmit as much of the deflection.

 

Hey all;

We have some keen minds here. I mean, where are you gonna get this kind of stuff, eh?

I'm with Dave on this one. I know that piston knock back is a reality under the right circumstances, but the theory David espouses is much more common. In fact, it happens at the Glen all the time.

I noticed the same thing a couple weeks ago. I'd brake hard for The 90 and then be a bit mushy for the Inner Loop, and then have a REALLY mushy pedal for turn 6. It happened no where else.

In fact, this place is a great one for brake problems. I had the pedal go to the floor braking from 200' into The 90 years ago. Sort of really sucks when you have none at this track!

To that end, I have developed the habit of tapping my brakes before the braking zones. Since there is usually no where to go, and my car would look truly awful with blue pinstriping, it seems a good habit to have. Sort of makes passengers wonder what the hell I am doing, but they see my point when I explain.

Anyway, the reason I know what it was when it happened to me last event is that I fixed it. Over the winter I had accidentally biffed the brake bias ****, and so having no idea where it was, I zeroed it. I have been slowly cranking more in as I get my chops back this spring, and I cranked in enough extra rear brake after the recent mushies to make the problem disappear. Less front brake effort. Less heat.

That's my theory. Maybe it's time for more cooling too?

 

From my experience, on my 944S, when the pads wear past 2/3rds of the way, the piston will back off (like three turns at VIR but not in others). Its one or two stabs from getting them back but its certainly not desirable and its certainly not in every turn. But by the chance that it is a wheel bearing, I would inspect them very thoroughly. From guys I know that have lost a wheel due to a bearing (snapping a hub), the tell tale sign is the brakes backing off. That's on a Honda, quite possible the bearings on your car are overbuilt in comparison.

I agree with Mark on the fluid. Pad fade from boiled fluid from my experience never really comes back and although the pumping gets pressure but its not a solid pedal. JME though.

BTW, having driven on a bad front wheel bearing on my 944S, I could hear it howling. Having raced on a couple on my CRX, I never knew they were bad by driving the car, just once I got it off the ground (don't think I could hear them in there regardless).

 

Adam,

You raise an interesting point and something I neglected to mention. Sorry. The rotors were nearly gone and the pads were half gone. I usually throw away pads once they get to the halfway point but since I am going to redo the brakes in the next month, I decided to use them up. I've never ran with that little of pad before. By the end of the weekend, I only had a third of the pad left.

I've boiled my brake fluid before (that's why I switched to Castrol SRF) and you are correct, they never really came back.

George

 

George, are you running stock 993 brakes or Big Reds? Do you have the plastic scoops underneath? What pads are you running? I have Big Reds the scoops RS ducts and Pagid Orange all around. My buddy is running the same except Pagid Black in the back and no RS ducts. We were at Waterford last week and my brakes were awesome and his kept getting mushy! The previous owner ran SRF in the car for almost the whole season. I run Super Blue and bleed after every event. With little mass between pads and rotors sounds like fluid break down. I will be curious to know. My buddy has new pads and rotors though, dont know whats up there?

 

CMMTracknut,

I've got the stock C2 setup (no Big Reds) and I use Hawk Blue's. The only thing I've removed is the thin metal shield behind the rotors. I do have the plastic little scoops underneath but don't have any ducting in the fog lights like some do.

I've never had this problem before. I do run slicks and have a fairly beefed up suspension 650/800 lbs springs. Also, I had a passenger for every single session I did, I guess that adds anywhere from 150 to 250 lbs to the car.

I seriously doubt it was the fluid since it only happened in one area and only in that area, otherwise they felt great. A half a pump on the pedal would get them back to normal in the problem area.

Here is a link to the video I made at the DE. I’m having the problem at the braking zone that’s at the 1:35 section of the video. The last time I use the brakes before that area happens at the 1:00 section of the video.

993MSR935.wmv

Mark, do you were I can find anti-knockback springs for my calipers? I’m probably going to rebuild them and wouldn’t mind the additional security. I wonder if they make pad changes more difficult.

George

 

George - what turn? Little Bend? Could be any number of things including pad fade and knowing that track that's the most likely place for it to happen irregularly. Just guessing but I wouldn't expect knockback or a loose bearing to cause it to happen just there - There are a lot more likely places on the track for that to happen IMO... Still need to flush that fluid you cheapskate