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A8 / S8 (D2 Platform) Discussion Discussion forum for the D2 Audi A8 and S8 produced from 1994-2002

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Old 09-26-2003, 08:02 AM   #1
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Default Why do you think a brake pedal gets soft after repeated hard braking?

So during one run session at Road America, I used my brakes VERY, VERY, VERY HARD.

I drove deep into the corners on the straights and really cranked on the brakes. They worked incredible. Yes, they did.

After I did that for two laps, I figured I could control the car better with less brakes, and I could actually go faster with less pressure and slightly earlier braking points.

However, my brake pedal started getting soft, i.e. increased brake pedal travel to get the same braking effect.

The brakes were hotter than ever, and when you go back to the class room, people ask the same thing "why is my pedal getting soft"?

Everybody's answer is "you have air in your system", which I found very hard to believe. There are not as many techies at a club event as you might find here.

So I didn't do anything to my brakes, and they came back the next run session, I figure it was because it had time to cool down.

My instructor asked me on my next run if my brakes were fine, I said yes, and he asked if I had stainless steel braided lines. I told him, only in the front, the rears were still stock (although I have braided lines for the rear in the trunk of the car [not installed], but I guess that doesn't count).

Then I thought about it. With all the heat that the brakes are creating on a course like Road America (my tire pressures would go from 41 PSI to 51 PSI during the course of one run group, 25 minutes), the brake lines could be expanding under intense pressure and heat during hard braking.

Then the car sits for 2 hours and it cools down and so do the lines. The brake pedal feel returns (but not like it is on the street).

Then I drive home and leave the car overnight, now the pedal is very, very firm again with the track pads on.

So, do you think it's the brake lines that expand under extreme heat and pressure?

If so, now I really understand why great race car drivers use the brakes as little as possible, keeping their heat as low as possible, so as not to have these problems.

My instructor said that great drivers can take street cars on the track with stock brakes and not overheat them because they know how to conserve them (and carry momentum through corners).

pw

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Old 09-26-2003, 08:24 AM   #2
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Default It makes perfect sense. Under hard braking you probably got some extreme heat in there.

I wouldnt be suprised if the calipers and their pistons expanded also.
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Old 09-26-2003, 08:30 AM   #3
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Default I think you answered your own question;)

Get the SS rear lines in and whilst your at it, flush the lines with SRF.
Geeash $100 for hydraulic fluid? Should provide about two grins a day for a year, or about 14 cent a grin.
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Old 09-26-2003, 09:16 AM   #4
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Default What kind of brake fluid are you using?

Maybe your brake fluid is boiling? There are several fluids available with dry temp ratings up to 590 degrees.
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Old 09-26-2003, 09:18 AM   #5
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Default I was using ATE gold.....good stuff, I don't think it was boiling

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Old 09-26-2003, 10:05 AM   #6
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Default Some brake cooling ducts might help

Worked for me on my old M3. However it sounds like you're not experiencing brake fade, just expansion of the hydraulic lines, so maybe new lines will do the trick.
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Old 09-26-2003, 10:15 AM   #7
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Default Your brake fluid is boiling.

That brake engineer guy at StopTech spells it out .. you need to change your brake fluid before and after every track day.

Once you boil it, it's future boiling point is 170f lower than it was before.
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Old 09-26-2003, 10:21 AM   #8
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Default If you dont change fluid before track day, all the water in your system boils nice n eaaasy.

unfresh brake fluid boils at least 170f lower than if it's fresh, due to water.

Check "dry" and "wet" boiling points of your fluid.

Dry = new, Wet = old.
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Old 09-26-2003, 11:48 AM   #9
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Default I don't necessarily agree with you

I might of had fluid boiling, but I'm not convinced of it. I think the components overheated and the flexible brake lines had flex in them, causing the brake system to be "compressable" versus "non compressable".

But you could be right.

I use ATE fluid and I did a complete change out four months ago.

Thanks for your opinion, will have to think about it some more.

Paul
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Old 09-26-2003, 01:32 PM   #10
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Default I would think the fluid is boiling ...

Relative to JKay's posts, I have two books of Carroll Smith's (I believe Carroll passed away recently ??) at home and will try to dig up some diagnosis info for you over the weekend, as well as reference Circle Track magazine (900hp, two tons, short track = brake problems). I'll also keep in mind the input of others' on possible causes.

In general, I think boiling fluid is the obvious answer. Usage of SS brake lines could help, I don't think it would be the root cause.

However, I am also waiting for Randy's input :-).

The heat build-up of slowing down a fast, two ton A8 from the long straights of RA will be extreme. Your brake modification is going to create heat that has to be transferred off to somewhere ... air, fluid, rim, tire, etc.
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Old 09-26-2003, 01:32 PM
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