Master cylinder??

[Fred1956]

I was at a race on Sun. riding the '04 EC300 & while whistling thru a rock garden section I tapped the rear brake and NOTHING!! Like the hose was cut into or the pin fell out. No resistance what so ever, after I unwound from the trees I plowed into, I hustled on down the trail for a couple of miles using front brake only & as I came to a nasty downhill section dreading the front brake only, I sneaked a try at the rear pedal & it was back! Worked great for the next 30 miles to the finish. I'm puzzled as to what could have happened. Do I need to rebuild master cylinder? Flush the system? Anybody else had this happen? Thanks