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Front brake locked up

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Anyone have any ideas on why a front brake would lock up?  My son was racing his '04 KTM 65SX last Sunday when he fell over (on the right side) and his front wheel locked up.  It's the brake.  I pulled the caliper apart and used a C clamp to push the pistons back in.  Didn't see anything obvious down there.  Pulled apart the master cylinder and it looked pretty clean.  I actually rebuilt the guts to the master cylinder over a year ago.  I then spent an hr bleeding the brake (always takes me forever to get the air out) and it locked up again.  Any ideas?

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Bent lever. There needs to be a tiny bit (at a minimum) of space between the lever and the master's piston. If they are touching it can prevent the brake from completely releasing. Also, it is possible (though n]very unlikely) the caliper or one of the slider pins is bent.

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If not that, make sure the check valves in the master cylinder are free. Had that happen to me going down the freeway on a virago once, m/c needed a good cleaning to get it going again. Try bench bleeding the m/c, it should go better for you. Loop the output back into the reservoir until you get all the air out.

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Yup, like the last two posts implied, "check the master cylinder".  As you most likely know after the brake is applied it has to spring back and expose the internal port to the main reservoir.  If it doesn't then when the fluid heats up instead of it expanding into the master cylinder it'll slowly apply the brake.  A bent brake lever, sticking piston or anything that would prevent the master cylinder piston from fully returning needs to be looked at.

Eli Tomac's mechanic should be really good at spotting this by now...  🤣   :rant:

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Thank you, I hate to throw parts at it since I didn't see anything that looked bent, but I think an aftermarket master cylinder rebuild kit is relatively inexpensive.  I'll try that first.

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