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why do my brakes go long?

M

Mario149

New member
scenario:

on a track, the car and brakes are warmed nicely, i do few fast laps, not much brake fade, everything hunky dory. come in for 15 mins rest and a cuppa tea, the go back out. however, on the first lap or so after i've gone back out, the brakes are really long but then go back to normal. why/how?!
 
I'd suggest that the heat has boiled the brake fluid whilst the car has stood, the movement of the car back on track has cooled the brakes and fluid.

garyw
 
Never put the hang brake on when you have been on track. Just leave in gear with the handbrake off to stop heat transfer into the fluid from the discs
 
ORIGINAL: mt0859

i'm always careful with that :)
Click to expand...

I wouldn't worry if I were you - having the hand-brake on or off won't make any difference to the temperature of the brake fluid as the hand-brake brake shoes are located in the "hat" section of your brake discs and have no contact with the brake fluid at all as the hand-brake is cable operated.

What might happen if you do put the hand-brake on whilst your stopped is that the little hand-brake brake shoes might become stuck to the inner surface of the brake disc "hat" if it's very hot - but I doubt this would happen.

The reason that the pedal travel was excessive when you went back out is due to the brake fluid having cooled down while you were on stop. You become used to the brake travel on a car extremely quickly (as you do with the bite on a clutch) and would therefore notice that the "bite" on the brakes comes further down the pedal travel when the fluid has cooled. When you were out on the track the temperature of the brake fluid would have shot up and therefore the "bite" on the brakes is much higher up the pedal travel because the brake fluid has expanded.

Having said all of that I'm sure that a 911 should be designed to cope with these situations as (one would assume) that if you just kept on cooking it 'round the track you'd eventually end up with almost no pedal travel and subsequently no brakes..?[&:] It might be worth getting the level of your brake fluid checked - or even getting your brake fluid renewed. Has it been changed recently? It'll need to be changed more regularly if you've got a habit of thrashing the car 'round tracks as this process will happen to your brake fluid more often than normal and it'll absorb more moisture when it cools - ie. you'll have water in your brake fluid which will expand far faster than your brake fluid and also cause corrosion.
 
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