I had a problem yesterday with my K1200S brakes. Usually, when the bike is started, the "BRAKE FAILURE" light flashes once a second, and stops flashing when I have gone a couple of yards. Yesterday, it flashed 4 times a second, and the brake servo was not working. This is "limp home" mode, where the brakes are not assisted, and ABS is not available. I called BMW and they told me that the reason for this was that the computer could not complete it's tests on the braking system. The usual reason for this was that the rear brake was jammed on, or if not, the rear brake switch was on, fooling the computer to think that that the rear brake was on.
I checked the small rear brake switch, and as the guy suggested, there was a small stone lodged in the metal strip of the switch. Once it was removed, the bike returned to it's normal operation.
The tech told me that this was a common fault, and BMW really should have put a rubber boot over the switch.
2009 K1300S in Grey - All factory options & Titanium Akrapovic exhaust
Previous bikes :- 2004 K1200S (RIP), 2002 CBR600F1, 2000 CBR600