anyone have this problem? rear brake mc pulling apart

2009 yz450f . i have had this happen three times now, twice in the same day. the plunger/washer/circlip come out of the rear master cylinder while riding (still attached and dangling from the rear brake pedal).

thew first time i thought it was a fluke, hit a rock or something but twice last sunday suggests a problem. circlip seemed normal as did the groove it sits in. does anyone know of an issue that would cause this?

The only thing that comes to mind is that your brake fluid is overfilled and/or contaminated with water, but heat expanding the fluid to the extent that it would pop the clip out should at least cause some noticeable brake drag before that happened, I would think.

thank you Greyracer, I was hoping you would have an opinion. perhaps your right (still hard to understand that heat could cause that to blow out). i bleed these brakes frequently as my (pro level) son has a tendency to boil the fluid alot. i know that both times this happened Sunday, i had filled the MC to the point that it overflowed when i dropped the rubber in. I assumed the rubber would compress if needed , didnt think i was overfilling. your thoughts?

If the top seal/bellows was the right shape when it went in, what you did sounds right.

The rider's not yanking the lever with his foot or something, is he?

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