As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
THE AIRCRAFT IS EQUIPPED WITH A DUAL VACUUM SYSTEM. THE SUCTION GAUGE FILLED WITH CARBON DUST DURING A VACUUM PUMP FAILURE. THIS OBSCURED THE PILOT'S INDICATION OF A LEFT OR RIGHT VACUUM PUMP FAILURE (RED BALL), SINCE THE GAUGE STILL SHOWED VACUUM IN THE GREEN ARC UNTIL THE SECOND VACUUM PUMP FAILED AND THE GAUGE WENT TO ZERO.