stevensa

I recently purchased a 2002 z06 and began taking it to HPDE events shortly after. This is the first car with ABS that I have ever had on a track. I have noticed that at the beginning of an event my brake pedal is firm and responsive about a 1/2" from the max height, and as the day goes on it slowly sinks down. usually by the end of the day the pedal must travel about 2.5" before I begin to get pressure. At that point the brakes act totally normal, and I have never felt as though I could not stop to full potential. After the day is done it usually takes quite a bit of normal braking cycles to get the pedal back to normal, but it eventually gets there.


The car is basically stock with xp10 front, xp8 rear and fresh ATE Super Blue for every event.

Is this something that is normal? It only happens when I am using the abs a lot. If I drive slower and dont really ever have the abs kick the pedal stays in the same spot.

autoxer6

Sounds like pad taper. They wear more on the leading edge. You can visibly see it if you look at the pads. Street driving helps to square them up again since you aren't puting much force on the pads like you do on the track. Another trick is to flip the pads so the thick part is now the leading edge.

Just my 2 cents.

ghoffman

The stock slide rail calipers spread more and more the hotter they get. It 400F the aluminum loses 25% of it's stiffness, and at 600F you lose almost 40%. Then as they spread, the pads taper.

wtknght1

Everything that's been said is correct. These 2-piston squeezer calipers are good for the street and not bad for short usage on the track, but as the day goes on and the pads wear, you will get a longer pedal. That's why we start races with new pads and rotors, 'cause by the end of the race, the pedal is L-O-N-G.

The Carbotechs don't seem to suffer as much as the rest; and it's more of a weight and poor caliper design. The C6 calipers are better than the C5s because they have a thicker outer "arm" so they spread less - better overall pedal feel. We've been telling the Comp board this for years (in T1) and hopefully we'll be able to bolt on some serious brakes for next year.

Jason

Pretty normal. As long as you can engage the ABS before you run out of travel, it's good.

drivinhard

Before the cooling mods, I usually had to do a quick bleed about 1/2 way through the day to keep a decent pedal. Now with the cooling bits, I can get the better part of day out of them with a decent pedal. Still need a quick bleed before day #2.

I have run the higher dollar fluid and ATE, and I can't tell a difference vs ATE. $10 mushy pedal or a $40 mushy pedal

ghoffman

So what does that tell you? It tells me that the mushy pedal is not due to the fluid boiling, but because of the caliper getting soft with temperature.

wtknght1

The Castrol SRF is the best fluid I've tried to help this situation too. Motul was good (and much cheaper) but I had to change it more often...so the Castrol to me is a better deal over the season.

stevensa

Thanks for all of the information, very informative.

Drivinhard - What cooling mods in particular did you do?

namalaw

I'm having exactly the same problem. I've changed brake pads, calipers, wheel bearings, and installed the quantum brake cooling kit. I thought I was boiling the fluid but my rotor temps were under 200 degrees. What I came to realize was that I had trapped air in the ABS module.
Are you bleeding your brakes using the TECH II ABS module bleed procedure? If not, I'd give that a try, as it'll get rid of any trapped air that's in the ABS module. If you don't have access to the TECH II, then you'll have to take it to your dealer. Supply the fluid of choice, and specify that you want them to do the TECH II bleed using the ABS module bleed procedure.

Namala

drivinhard

While I'm sure that is going on, a quick bleed of either brings back a firm pedal, so the fluid is obviously getting boiled.

DRM front ducts, Quantum hub ducts

One "trick" I tried recently in regards to that, was to open BOTH ends of the circuit at once (ie, LF, RR, etc) with the motive putting + pressure on the system, and then run the ABS pump (TC with the car up in the air on the lift) and it worked great. Previously with air somewhere in the pump/valve maze, if I did the same with only one end of the circuit open (ie just the LF, or just the RR, etc), there was no change.

Aardwolf

Sounds like pad taper. Pop an inboard pad out and check quick. The leading edge should have less material then the other end. Swap them from inboard to outboard after each track day.

Poor-sha

Can you please elaborate on what you are doing here to run the ABS pump?

namalaw

X2 Please!

Aardwolf

The ABS on my C4 runs continuously if the front wheels are locked and the rears are turning. Apply light braking pressure so as not to over heat the rears while the wheels spin. A lift works nice for this but snow does wonders as well.

drivinhard

running the rear wheels with the fronts not moving (ie, turning on the traction control - ABS pump)

stevensa

Do you just stick it in gear and let it idle while bleeding the brakes?

You do LF/RR and RF/LR at the same time then?

drivinhard

stick it in 5th, give it a quick romp, let the TC catch it

yeah in order to bleed the pump fully, I had to open both ends of the circuit (LF/RR, etc) at the same time. normally, you don't have to do all that. just stick the motive on it (or 2 man) and away you go, corner to corner.

stevensa

Do you continue to leave it in 5th with the wheels spinning and the TC active while bleeding, or does the quick romp with the bleeders open under motive pressure do the job?

drivinhard

pretty quick, doesn't take much

this is pretty stable on a 2 post lift, if you are doing this with jackstands, etc...just be careful!