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).
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.
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.
'03 MB CL55 Kompressor Horizon Blue Metallic
'03 Volvo XC70 Ruby Red (wife's car)
Audis of the Past:
'00 Silver Audi A6 4.2
- Lowered stall speed for the pullin' like a freight train effect.
- TozoA8 super ATF
'98 Ming Blue A8 (April 23, 2003 - lease termination, a sad vehicular day in the Bragg household)
'98 Audi 1.8T quattro (Tropic Green)