a freak thing

so today i was racing my 2012 kx250f in a harescramble like normal. so at the end of the first lap my back brake totally stopped working so i stopped and tightened the brake bleeder and pumped the brake it worked but at the same spot on the second lap same thing so did the same thing pumped the brake and it worked so kept going and the brake stopped at the same place so i pulled off and tried the same thing and it didn't work so i opened the brake resivour and it was empty found a guy with brake fluid and bleed.... then i worked fine. has this happened to any one? I'm very frustrated

sorry for bad grammer i typed this on my phone

You are losing fluid. Maybe the spot where you were at tipped the MC just right at the exact time to tip fluid away from the intake at the bottom of the reservoir. Have you looked for evidence of leaking?

Happened to me with my 11..... got on bike to go down to the starting line and brake seemed funny but I pushed the pedal and back wheel locked up, however out on the track when the bike is going faster with more momentum different story.... no brakes.... brought it back filled up reservoir and bleed a couple times and all was good. Since then nothing, with no signs of leaks.... I don't know where it went but it went somewhere at some point... I don't think it can just evaporate out ha

You are losing fluid. Maybe the spot where you were at tipped the MC just right at the exact time to tip fluid away from the intake at the bottom of the reservoir. Have you looked for evidence of leaking?

yes it was leaking out the bleeder on the caliper the bolt came loose and it slowly came out.

Another common problem is moisture in the brake fluid.  Over time the brake fluid will absorb some moisture.  When you are hard on the brakes, like in a race with downhills with switch backs, the brakes get really hot causing the moisture to boil in the fluid making your brakes go away.  You lost fluid and replaced it when you left the bleeder loose the first lap, but your original problem was probably moisture in the fluid.   Best to replace brake fluid often and not keep bottles of it lying around to long because just being opened the fluid starts to absorb moisture in the bottle too.    

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