Did a track day a couple of weeks ago. Ambient temperature was in high 20's. As the session progressed the brakes really started to feel spongy/fade. I belive the brakes are standard Sierra. Attached is a picture of the rear set up.




Anyone any ideas on what to do/cause?



Andy
Rescued attachment Rear Disc.JPG

Try rebleeding? initially. What spec of brake fluid?, were you using , you could try DOT5.

How old were the pads? bedded properly?
You could have glazed pads.

What color are the disc's ? if they are bluey coloured becareful beware of surface cracking this is hydrogen embrittlement.

Brakes are funny things, but not funny though when they don't work.

Are the fronts grooved aswell?? What pads you running??


As said try rebleeding. The brake fluid may have got hot. If your using dot 4 try dot 5.1, but flush out well when u change over. If you put dot 5 anywhere near it after 4 it will shag your seals up.

Don't replace your brake fluid with silicone - you'll be disappointed. DOT 5 is silicone, DOT5.1 is racing-quality traditional glycol-based brake fluid.

See here for a good description.

I'd try a good flush out with fresh fluid as a starting point, whether it's DOT3, 4 or 5.1.

David

Proper brake fade is caused by heat - nothing else.

If your brakes aren't bled properly sort it out but that effect would have been noticeable before you did this track day.

If it was brake fade then you can change to vented disks or bigger diameter disks - be careful of upsetting the balance if you do this, or you can get some cold air feeds into the brake/wheel area and try and keep them cooler that way.

Water in the fluid will also cause sponginess if the brakes get very hot - steam is quite easy to compress!

David

I Would say more cooling / airflow required,

Not examinined a GT but trust its rear engined where does the engine heat go ? if this is having to pass the discs as well wont be assisting. is ther anyway to get cool air to the disks.

What have you got your balance bar set at ?

regards

Agriv8

as "JAG" said.

its the heat..

my experience...july /August 04 and 05

with my Sccob in the Isle Of Man, i had severe brake fade and judder. this sorted itself out when it cooled down.

the year after, even giving the scoob more hassle, not a hint of Fade, no judder.

the difference was...

newer disks...but the same spec.
newer pads..but the same spec.
Changed to Dot 5.1 brake fluid.

Thanks for the replies.

Front discs are grooved as well, but I am not sure what the pads are as they were in the car when I bought it.

I had not experienced this before the track day or at previous track days. It certainly was a very hot day. It could just be in my mind but the brakes don't feel as good as they did before.

As for heat in the engine bay, it is very hot and there are brake lines in there too.

Balance bar was pinned for SVA (still is), but as I said no problems before this track day.


It supposedly has DOT 5.1 in it (previous owner does not answer e-mails), but again I don't know how old it is. Guess it is as good a starting point as any.



Andy

A different pad material will probably help - trackdays need harder pads if you want to push it to the limit. It is a good idea to have 'road' and 'track' pads and change them accordingly.

Try Greenstuff or Redstuff for a start

It may be a combination of fluid and pads. As has been said, for racing and track days you will certainly need a harder pad otherwise you will run out of brakes very quickly. You need to check that your brake fluid is of racing quality and even if it, is it is worth changing it in case there is some moisture in it. With good fluid and the correct pads you can run all day at racing speeds and braking without problem with the brakes.

John