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Brake problem on track
madteg - 21/10/13 at 07:52 PM

I have a good pedal most of the time but when brakes are hot on track, the pedal goes down about 2" and then its hard again. This is a little unnerving, Have tried different types of fluid. When brakes have cooled down the pedal comes back to normal.

NigeEss - 21/10/13 at 08:02 PM

Sounds like brake fade.

What pads are you using ?

Bare - 21/10/13 at 08:04 PM

Gas from boiling fluid (water usually) is compressing... Flush and refill the Brake fluid BEFORE Each race.

madteg - 21/10/13 at 08:09 PM

Running big discs (297mm) with 4 pot Brembo calipers, ferodo pads

yellowcab - 21/10/13 at 08:17 PM

What pads exactly? ds2500 are only really a fast road pad, very entry level track day

Anything more full on than that, you'd want ds3000 compound or higher to eliminate any fade

snapper - 21/10/13 at 08:23 PM

Brake fade does not give a dropping pedal
I go with Bare's view
Brake fade gives firm pedal but with much less slowing
Dropping pedal different problem

madteg - 21/10/13 at 08:27 PM

Will try new fluid and get some hot pads, thanks guys. Kev.

britishtrident - 21/10/13 at 08:50 PM

First thing it is fairly safe to say it isn't pad material related.
It isn't brake fade, brake fade is an increase the pedal effort needed to slow the car due to the friction coefficient of the brake changing with temperature. With brake fade the pedal travel doesn’t change but the push required on the pedal increases progressively tup o a point where the pad friction coefficient falls off a cliff.


Long pedal when the brakes are hot is either caused by fluid boiling or if you have drums on the rear the drums expanding.
Fluid boiling causes a very sudden increase of pedal travel.
With drum expansion the increase in travel is less sudden.

Brakes have to get very very hot for fresh brake fluid to boil, with fluid boiling normally the first thing to look for is a binding brake. Check the brake temperature on each wheel. If the brake fluid reservoir is overfilled it can cause over heating, also check the brake pedal has a return spring. Jacking out the master cylinder pus rods often leads to problems, the master cylinder piston must always come fully back against the end stop of the cylinder.

Calliper or handbrake problems are often the cause of binding brakes, check the pistons and pads are moving freely in the calliper and with Sierra floating callipers check the callipers is moving freely on the guide pins and the handbrake cable has enough free.

If a calliper appears to be seized partly binding with no other cause the problem can be caused by the inner rubber seal hardening.

unijacko67 - 21/10/13 at 09:13 PM

When you say the pedal is soft then hard again do you mean it comes hard after one pump or when its cooled down as a slack wheel bearing or slightly warped disk will rattle the pads back into the calliper so the first pump is pumping the pads to the disc, second pump and it will work.

madteg - 21/10/13 at 09:23 PM

After 1/2" of travel the pedal is hard. When brakes are hot on track the pedal travels about 2" then i have a hard pedal. The same pressure is needed to stop the can.

unijacko67 - 21/10/13 at 09:32 PM

Sorry ignore my ramblings in that case.

CNHSS1 - 21/10/13 at 09:36 PM

Pad knock off under hard cornering? Not present under normal driving but when leant on, wear/slop/poorly adjusted bearings allow small amount of disc movemnet oushing pads back giving a longer pedal until the extra piston movements taken up?
On some classics used to circuit race, can even be stub axle flex

madteg - 21/10/13 at 09:40 PM

Now this could be it i like it. Thanks.

ashg - 21/10/13 at 11:11 PM

I'm guessing your master cylinder is not the highest point in your brake system. If that is the case then you need one of these on each line.


http://www.rallydesign.co.uk/product_info.php?cPath=562_596&products_id=4081

britishtrident - 22/10/13 at 06:23 AM

quote:
Originally posted by ashg
I'm guessing your master cylinder is not the highest point in your brake system. If that is the case then you need one of these on each line.


http://www.rallydesign.co.uk/product_info.php?cPath=562_596&products_id=4081


The height of the free surface of the fluid in the reservoir rather than the height of the master cylinder is what matters even then the effect is tiny when the car is being driven.

britishtrident - 22/10/13 at 06:34 AM

quote:
Originally posted by CNHSS1
Pad knock off under hard cornering? Not present under normal driving but when leant on, wear/slop/poorly adjusted bearings allow small amount of disc movemnet oushing pads back giving a longer pedal until the extra piston movements taken up?
On some classics used to circuit race, can even be stub axle flex



Pad knock back used to be common on the old Formula Fords and similar formula but unlikely with modern wheel bearings and floating callipers as used on Sierra it is much less likely unless something is wrong such as a really knackered wheel bearing or a warped disc.

One measure to counter pad knock back is fitting a residual pressure valve as mention by ashg another is fitting very light light springs inside the callipers, both of these cures are really last ditch measures.

eddie99 - 22/10/13 at 07:27 AM

Id say brake fluid, symptoms don't lead to a pad problem

It's mega expensive but if you can try the castrol srf, IMO it's the best fluid and will eliminate a fluid problem

We were having to flush through regularly using the willwood high temp stuff, the castrol Is another league