I have a dead master cylinder on an M5, and it seems that ATF had been mixed with the brake fluid...
'85 Euro 635csi Race Turbo, megasquirt, Group A
'92 525i Touring 5 speed, 3.46 diff, UUC 8.5lb flywheel, soon to be turbo.
'02 Subaru WRX Wagon with STI springs, bigger sway bars
my son put power steering fluid in brake fluid res. once and it it kilt it dead! eats the seals.