Brembo brake failure

[KYHU]

My bike is a 98 KTM EXC that I bought used but in excellent condition 4 months ago. I posted it here rather than in the euro forum, because I think it can be useful for everyone.

I was riding with a friend yesterday and I had a MAJOR front brake failure. It was at the end of a pretty steep and lond paved road. (going downhill) We weren't riding particularily hard. I must add that, in order to preserve my rear tire on a paved roads I brake about 90/10 percent front/rear. The front brake was fine and the feel at the lever indicated no sign of overheating. After one of the last turn I let go of the lever in the straight and when I grabbed it again before the next curve, I had no more front brake !! The lever was moving all the way to the handelbars without slowing me at all. :scream: Thankfully I was going slow and was able to stop with my rear brake.

I inspected the bike and I could push the lever up to the handelbar with my little finger. The lever was moving effortlessly (like if there was no oil in the system) but the brake pads were not. It was because the oil was dripping out of the drain screw on the caliper. Here's a pic where you can see the caliper and the oil stain on it : Brembo caliper
After the brake cooled down it was working again as well as before, but with less oil in the system. Before you ask, I checked the screw. It's tight.

The cause of this accident is probably overheating. But I've been using and abusing my front brake since I've had this bike and it had always worked perfectly. I'm probably going to give a call to the distributor of Brembo and send the brake assembly to the fabric, to be assured that this won't happen again. So let my experience be useful to you and double check your brakes before your next ride. Because having your front brake go ON/OFF in a split second is scary.

I still can't believe it happened... :silly:

Your commentaries are welcome.

david

 

[Jimmy!]

I'd pull the bleeder screw out & make sure there isn't any junk under it. It's a pretty simple device, not much to go wrong. The tiniest bit of debris can make it seep though. If there's any scoring/scratches on the tapered "seat" area. I'd replace it. Also take a look at the seat in the caliper & look for damage there. Inspect the caliper body for hairline cracks in the area of the bleeder screw as well. If all this looks good, bleed the system & close it up good & you should be O.K.

 

[KYHU]

Thanks Jimmy, but I don't know if I should check it myself rather than having it done by the factory, or someone really competent. Because if I check it and find nothing wrong, what do I do ? Ride again with the same caliper ? No way, I don't want to take that big of a risk.

david

 

[cujet]

Brake calipers are ultra simple. It is possible you boiled the brake fluid due to moisture in the fluid. I suggest using at least dot 4 fluid from a new container. That way you are sure there is no moisture in the brake system. Bleed the system fully and fix the leak!
Chris

 

[David Trustrum]

Yeah the leak is pretty damning. Dirtbikes have pretty puny brakes, which is fine as we don’t put much long-term pressure on them. The problem is when we put them in a high traction arena & add speed, like on tarmac.

You will notice road bikes have bigger discs & callipers to deal with the prolonged heat (& extra weight). Overheated pads can delaminate separating from their backings which is what I first thought your problem was. Also old fluid can give similar problems.

But if it’s leaking it ain’t right.

 

[KYHU]

Thanks to all for your replies. I'm gonna have a serious look at it and I'll tell you what I found.

david

 

[ktmboy]

KYHU- KTMs' take a different brake fluid- my '96 calls for DOT 5.1, but check your manual. Using the wrong fluid can cause fluid boil! The 5.1 is harder to find, but the local Honda dealer had it here in my neck of the woods.(Motul).I would suggest completely replacing all the fluid with whatever fluid your manual says your bike uses, and then giving the system a good bleed.

 

[KYHU]

ktmboy : on the master cylinder it's written : use only DOT 3/5. On the manual they say: never use DOT 5 brake fluid because it's silicon-based. They say to use Shell Advance Brake DOT 5.1. But isn't DOT 5.1 silicon-based ?

Till now I used AP Racing 600 (for my DH mountain-bike). Not intended for highway use, so it's not categorized by the DOT. But it's maybe the best brake fluid money can buy, that's why I used it.

So which oil do you think I sould use ?

thanks

david


Edited to make a correction : actually, DOT 5.1 (also called Super DOT 4) is the designation for recently developped brake fluids, that have evolved from the DOT 4 brake fluid but that have a higer boiling point. (446F for straight DOT 4 and 500F for DOT 5.1 or Super DOT 4) DOT 5.1 brake fluids are not silicon-based.

There's an excellent article about brake fluids here .

 

[KYHU]

Originally posted by KYHU
So which oil do you think I sould use ?

Ok, I'll do the answer, too. :cool:

To simplify the problem, let's suppose that you want the best performance from your braking system, cost being not an issue. You have two choices : DOT 5.1 or Racing brake fluid.

The difference between the two is that in general the various brands of racing brake fluid have a higher DRY boiling point than DOT 5.1 fluid. But racing fluid has a much lower WET boiling point. ("wet" means that the fluid has a fixed percentage of water in it as opposed to "dry" which means that there's no water in the fluid) Practically, that means that if you decide you want the very best performance and choose Racing fluid, you'll have to change the fluid more frequently (after every race) than if you choose DOT 5.1 fluid.

So my recommendation is that you choose the best DOT 5.1 brake fluid you can find, and that you change it at least twice a season.


david

 

[ktmboy]

You say you bought the bike used. I'm no expert on brake fluids, but if I had a failure like you did, I might be concerned that maybe someone had topped off with the wrong (maybe incompatible) fluid. With a system that holds such a small amount of fluid, it wouldn't take much to spoil it.
BTW, I do appreciate your post; as a KTM owner myself. I will definately be keeping a closer watch on my calipers.
I use the Motul fluid because it is specifically manufactured for motorcycles.
Hope you figure out what caused your boil-over.:D

 

[KYHU]

ktmboy : I think I finally figured out what happenend. You say that maybe someone mixed incompatible fluids. I have thought about that possibility, too, but it's impossible. DOT 3,4 and 5.1 are all compatible. And the DOT 5 fluid is of special color (purple) and has a warning on the bottle saying that you must not mix it with any other fluid. I've seen the color of my brake fluid, and it has not been mixed with DOT 5.

My idea of what happenend is this : the previous owner has never changed the brake fluid, or has put some fluid contaminated with water in the system, which brought it to a dangerously low boiling point. The water accumulated in the fluid boiled, causing a "vapor lock", blah blah blah...That's my take on what happened. One way to be sure that it is really what happened is to test the fluid to determine its water content. Maybe I can find someone who could do that for me, but probably not.

I also think that the liquid that was dripping from the caliper was mainly water, because now that it's dry the spot were the liquid was is not oily at all. I guess that the high temperature of the water, its low viscosity (compared to the brake fluid) and the pressure in the system are the factors why some liquid dripped past the drain screw. Tomorrow I'll bleed the brake and probably confirm that the caliper is ok.

Does my explanation sound reasonable ? :think

david

 

[KYHU]

Here's a chart that shows you why it is important to change your brake fluid regularly. Knowing that in one year, you have between 2 and 3 percent of water in your braking system you can easily see how dramatically the boiling point of the brake fluid falls. In other terms, it's MUCH better to buy brake fluid of a lesser quality and changing it very regularly than buying racing fluid and never changing it.

david

 

[ktmboy]

David- thanks for all the information. I've noticed brake fade on the rear with heavy usage, and I know a little preventative maintainence is probably due. Thanks for the reminder, and I hope you're right about water in your fluid. It sounds like an easy fix.

 

[KYHU]

I bled my brakes (I used Motul DOT 5.1 brake fluid) and checked the drain screw. As I thought, the caliper are not in cause. I'm almost 100% sure that the accident was caused by bad brake fluid.

david

 

[ktmboy]

:) Now go out on a good, long,brake boiling ride and let us know the results!:)