The same issue happened to me, my friend (owns a E92 M3) and another guy with the E39 M5. Just saw this thread and am relieved to see its a fairly common problem. We were at VIR where the temp reached close to 88' and mad humid..probably hit 90' + at the track.
Maybe the next time around I will use a shop towel and a zip tie around the reservoir cap.
Last edited by checkmate; 05-10-2012 at 11:01 AM.