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FRONT MASTER CYLINDER PROBLEM

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I made the unfortunate decision to let a nephew on vacation borrow one of my motorcycles. It's a 2007 KX 250F with hardly any time on it.

It's the one that sits in the garage and never gets used.

Well he crashed and now the front brake is a sponge.

I bled it a bunch of times and don't see any air bubbles however it never seems to pump up. I checked for cracks and leaks and don"t see any. 

Any ideas ?

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Maybe time to rebuild the master cylinder? I believe that's a two seal set-up, where if one seal is blown, it still sorta works. I've read reports of people with the same symptom: bleed, bleed, bleed, but doesn't solve the problem. Not unheard of for this to happen, even on smaller hits. 

 

 

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It's easy for air to be trapped in the banjo fitting at the MC, especially if it's a 90° fitting. 

Try taking the MC off the bar and holding it straight up and pumping it a bit, or loosen the banjo just enough to rotate it so that the hose is below the MC, assuming it's not like that already, most have the hose over the MC. 

Check for a bent rotor, although that would probably be obvious. Check the caliper mount bracket and slide pins.

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Thanks ! I'll check that out tonight.....I did check the rotor, been down that road before..

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I had the same problem until i took the MC off the bars and bled it straight up.immediatly some bubbles came up and the problem was fixed

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Push the pads/pistons back into the caliper which will back bleed the system.  I do this every ride as it firms up the lever..............

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Go get an ink cartridge syringe and put a tube on it. Shrink wrap. Take master cylinder cap off. Suck in dot4, flick all bubbles out, put onto caliper bleeder, crack bleeder open 1/4 turn, inject it all, close bleeder, put master cap back on, done. Took you less than five minutes to do this. If this dont work, rebuild master and get new (or anneal the ones you have) copper washers.

Took longer to type this than to do on a sportbike with ABS

Good luck

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After weeks of farting around with this and two bottles of brake fluid I decided to rebuild the master cylinder. 

Problem gone... I'm guessing the ground may have crushed the brake lever, forced it beyond normal limits and blew out the seals... 

Thanks for all the suggestions... I tried them all !

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A brake levers "limit" is the handlebar...you just needed new rubbers.

Ive crashed hard on an r1 and laid onto right side. (Newish master rubbers...) but my ASV levers nor my masters rubbers break but the master cylinder broke where the brake light switch screws in making the switch rotate with the lever never releasing the switch, not activating light. So I bought a used r1 master and put those (newish) seals off my broken master onto it. Warrantied C5 levers, now new gold w/black letters. No probs. 

Rubber items ARE a maintenance item. They wont last forever.

Im glad youre stopping now. Use that syringe trick from now on.

Caliper piston seals next?

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