My brakes would always start fading and feel mushy after a few laps at local track events. I have flushed out my brake fluid and replaced it with ATE Super Blue Racing Fluid and that problem has now disappeared. The ATE fluid has a boiling point 122 degrees higher than standard DOT 4 fluid which comes stock in the M5. Give it a try. It's an inexpensive way to improve the brakes on the M5.
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