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back brake failed today ...

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so i was out riding today descending down a steep hilly section of the trail and i hit the brakes once more and the back one totally fails, i triedd pumped the brake and it didnt come good, then i ride along for about 5mins without touching it and then it works again?!? but then after a little bit of light braking it totally stops working, so i guess the master cylinder needs a rebuild? im not really good with brakes and could do with a second opinion, the bike is a 97 exc300 the model with the brembo brakes. thanks guys.

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Moisture in the lines, bleed your brake. The moisture gets hot and makes your brakes fade. When it cools down your brakes come back.

Joe.....

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Were you riding/dragging the rear brake? I lost my rear brake on my 450 EXC over the summer after dragging the brake too much and overheating it. The brake came back after it cooled down. I ended up going with graphite pads as they don't run as hot as the stock pads and replaced the fluid.

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I would check the fluid in the master cylinder first. If you have a decent fluid level, change the fluid and bleed it well. If there is no fluid, buy the rebuid kit and you should be ready to go.

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Couple of suggestions.

First, when was the last time that your brake fluid was flushed/bled out? Brake fluid is designed to absorb moisture to prevent the brake parts themselves from becoming corroded. It needs to get flushed out periodically to renew the brake fluid and remove the old brake fluid that has too much moisture in it. I do mine at least once/year maybe 2 times/year if I am riding in more water/mud and washing it more often.

When brake fluid gets more moisture in it, the boilling point comes way down. I would suggest reverse bleeding the brakes for the best total flush. I typically use a brake fluid that is rated DOT 5.1 because of the higher boiling point of the brake fluid. Not many places cary DOT 5.1 brake fluid, you might have to order it. NAPA can get DOT5.1 for you. Motul RBF600 is a great fluid, but not cheap. Don't keep old bottles of brake fluid as they tend to absorb water after then have been opened. Do not get DOT 5 brake fluid. It is silicone based and can not be mixed with regular brake fluid that is DOT 3, DOT4, or DOT 5.1 rated. I would suggest that you get at least DOT4 brake fluid. The big difference is the boiling point. Bleed the brakes until the fluid comes out clear. Normally you will have to refill the master cylinder 3-5 times for this. I might also suggest compressing the calipers to reduce the amount of fluid left in them. I personally prefer reverse bleeding.

.

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Common problem on the KTMs.

They sell a rebuild kit for $31 at the local KTM dealer.

I've rebuilt mine 2x but it took a lot of hours to get it to this point.

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If you cook the brakes, the brake fluid obviously gets too hot.

I had some friends say that once you boil the brakes, replace the brake fluid.

Otherwise, it just boils easier the next time.

Now that I ride KTM, I noticed how the brakes squeek more than Yamaha.

It makes sense that the disk gets hotter.

Replace the fluid, before jumping to more expensive solutions.

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yeah the fluid was running low, and i have not bleed the brakes for ages, thanks for the info guy cause i would have just brough a rebiuld kit, iv got some dot4 motul stuff at home and ill replace the old stuff with that, thanks guys! ill see how it goes.

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yeah the fluid was running low, and i have not bleed the brakes for ages, thanks for the info guy cause i would have just brough a rebiuld kit, iv got some dot4 motul stuff at home and ill replace the old stuff with that, thanks guys! ill see how it goes.

Have you closely inspected the whole system for leaks? Anyplace where dust seems to stick to? Being a sealed system I would assume that the fluid has gone somewhere unless your pads have just worn down to nothing.

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yeah it looked like it had some moister in the line when i bleed the brakes, i put the new fluid in it and then, rode the crap out of the brake and it didnt go once! so im happy and it seems like its fixed, i think it may have go the moister in when i did a river crossing, when i pulled the resivor cap off the rubber grommet wasnt on properly.

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Brake fluid by design pulls moisture into it, needs to be changed occasionally . If your reservoir is marked dot 3 or 4, you can use a dot 5.1, it's a better fluid. DO NOT use dot 5 (silicone) unless that what's called for and is in it now.

I agree, KTM rears seem to overheat rears more often. It;'s that ready to race feature they talk about, LOL. Racers don't use brakes.

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