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125sx rear brake piston problem

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hi, have a '05 125sx, (not sure of exact year;i know it's relevant...) anyway, the rear brake piston keeps going out; i changed the piston repair kit mx times....even had someone in pennsylania who did a 'mod' on it; to change to a nissen type, or something like that; but it's out again...no rear brake !!!

what is the most definitive fix on this chronic brake problem???? i did use the ktm 5 fluid; not dot4...

james R

so calif

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If it was changed to the Nissen master cylinder then I think that would have been a complete master cylinder change. That and the fact that you rebuilt it several times would lead me to think it was something else other then the master cylinder.

How does the failure show itself?

The symptoms of a master cylinder failure usually start with a brake the works when you press the brake. It builds pressure, but then the peddle continues to drop as you hold pressure on it. Of course it could blow out the piston seals completely but I doubt that would happen over and over. It could have been a problem with the bore in the cylinder but that should have been covered by replacing it with the Nissen cylinder.

I would be looking for something else like a leak.

Armed with that knowledge can you provide anymore information, symptoms, how does it fail, etc?

Gary

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the 05 rear had a burr inside the MC that has to be smoothed out or get a different year master

ya know, some may have come this way. I had to rebuild my MC shortly after I bought my bike, but I haven't had an issue since (knock on wood). So, IDK that this is the case with all of them.

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yeah, i think my 125sx if the '05 MY...the brake just goes all the way down and has no holding...there is NO leak anywhere...when i put a new kit, it works fine, for 1 ride, the next time, there is no rear brake...i go back to the garage and take out the yz250 or crf450 then...

i sent the mc to some racing shop in east coast and they 'fixed' it but it still failed...not sure exactly what their fix was...

i really like riding the 125sx but the fix is getting old...what is the best thing? just get a 06 mc? i did read about the burr... it must be pre big burr...wonder if i could have smoothed it out....

i just want to have a reliable back brake...like the yz and crf...

not gonna get the mc repair kit anymore...gonna get the entire mc..prob is, what year mcs can i use in that 05 125sx? may be they are all similar..

james R

jjrho@roadrunner.com

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the 04-05-06 and maybe more, I didnt check the numbers except these years, are all the same part number. I would buy a used one from someone parting out a bike over at KTMTALK. New is $175.

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well, i was kinna confused 'cause i sent my oem mc to this guy and it looks like i got the same one back...not as a core exchange; and if it was some type of nissen mod, i thought it would hve looked different..didn't think he could just mod the inside.....

it's the mc alright...no leak, fluid level exactly same...

i may hve used a ktm fluid; althogh maybe i should have used the dot4 for the rear mc? i 'm not even sure wht the 5 fluid was for; ..for front? for clutch? some ktm brake needs 5 or 5.2 but the cover says 4 on most....pls clarify someone....tnx,

james R

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hi, well, i remember buying a ktm specific brake fluid....i dont' remember if i've used it to fill the 125sx rear mc...i know i used it on a 50sx for my kids...

so, are you saying that for the full size bikes, ktms...i should just use the dot 4 brake fluids, like the one's i'm putting in the yami/kawi/hondas???

james R

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Yes. DOT 4 is good. But it needs to be flushed out and all the seals replaced if you already put DOT 5 in it.

Read this carefully. I doubt if you got hold of any DOT 5 fluid but if you did this may be your problem and it is worth understanding anyway.

Again, what Mike said, DOT 3, 4, and 5.1 are all compatible.

DOT 5 is different. It is silicone based and not compatible with DOT 3, DOT 4, or DOT 5.1. Here is what Wikipedia says, see full text in link below:

"Using DOT 5 in a DOT 3 or DOT 4 system without proper flushing will cause damage to the seals and cause brake failure. DOT 5 brake fluid is not compatible with anti-lock brake systems."

Since there is a DOT 5 and a DOT 5.1 it may be confusing. In other words note that there is a DOT 5, and a DOT 5.1 and they are different and incompatible. KTM recommends DOT 5.1 in the manuals on the newer bikes but the reservoir may say DOT 4 on the casting (older recommendation) adding to the confusion.

Check out these links for further reinforcement:

http://en.wikipedia.org/wiki/DOT_5

http://stason.org/TULARC/vehicles/motorcycle-ducati/5-Should-I-use-DOT-3-4-or-5-brake-fluid-when-replacing.html

Gary

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