Did I Boil My Brake Fliud?

[jhjones]

So I did a track day on Sunday and on the 3rd session on the cool down lap my brake pedal went to the floor. I bleed the front calipers and pedal became good. Went back out and on the cool down lap the pedal went to the floor again. This happened 3 times.

Car is R32 GTST, R33 GTS25T brake calipers, Nulon Xtreme Super Dot 4 Brake fluid, Endless MX72 Brake Pads.

Edited June 8, 2016 by jhjones

[GTSBoy]

Would appear so.

[jhjones]

So the fix would better fluid? Motul RBF660? If that doesn't work do I need bigger brakes?

[Trex]

Motul penrite and trw are all around the 600 deg f. Get 2L to do a full flush.

[GTSBoy]

So the fix would better fluid? Motul RBF660? If that doesn't work do I need bigger brakes?

Yes and yes. And possibly yes. Although you might find that for track work simply rigging some ducts up to scoop more cooling air onto the brakes might be enough to keep stuff under control. Easier and cheaper to try, although just as scary if it doesn't work!

[jhjones]

Why does it only happen on the cool down laps? When Im hammering the car the pedal is good.

[V28VX37]

Why does it only happen on the cool down laps? When Im hammering the car the pedal is good.

Also, the point of a cool down lap is to not touch the pedal, no?

[KiwiRS4T]

Motul RBF600 has about twice as high a boiling point as your fluid so that should make a big difference. RBF660 is slightly better again.

[MrStabby]

Motul RBF600 has about twice as high a boiling point as your fluid so that should make a big difference. RBF660 is slightly better again.

Dont mix up Celsius and Fahrenheit... Off the top of my head its 280 (nulon) vs 315 (rbf600)

[KiwiRS4T]

Dont mix up Celsius and Fahrenheit... Off the top of my head its 280 (nulon) vs 315 (rbf600)

Good point. So Motul RBF660 is better but not twice as better:

Motul dry 325C Nulon super xtreme 284C

Motul wet 205C Nulon 183C

[jhjones]

I think ill give motul rbf 600 a go next track day. What i did was bleed the calipers till the new fluid came through. Should I take the lines off the calipers to drain them before i bleed it?

[Trex]

Nope. Just bleed away.

[KiwiRS4T]

And check your pad thickness - amazing how quick you can wear them down if you really try!

[Hadouken]

Tell me about it. Stupid intima sr's

[Komdotkom]

Can you get a Ti pad shim for the Sumitomo's? Pretty cheap and helps to stop the heat from the back of the pad getting absorbed by the piston and hence the fluid.

They can be made pretty easily with a set of quality shears and some .5mm Ti6Al4V which you can get online for about $40.

[Hadouken]

Is that even worth it? The amount of heat that radiates every where else will soon overcome your titanium shims

[Komdotkom]

I've found it makes a pretty big difference on cars that are under braked. From memory the Sumitomo's have aluminium pistons which transmit the heat very quickly to the fluid and boil it, so by adding a thermal shim you can reduce the amount of rapid heat transfer. This is why racing calipers (AP Racing et al) have castelated pistons to increase ariflow and reduce the amount of surface contact with the back of a very hot brake pad.

Obviously you can't get away from the radiant heat but this is absorbed at a slower rate, so in short sprints thermal shims should be fairly effective and for $40 you can't really go wrong.