What was found
(CAN) DURING MAINTENANCE, THE LT ENGINE WAS STARTED FOR A GROUND RUN. THE BLEED AIR FAIL ANNUNCIATOR FAILED TO EXTINGUISH ON BOTH THE LT AND RT SIDES. IT WAS ALSO NOTED THAT THE PNEUMATIC AND VACUUM GUAGES BOTH READ ZERO. SYS WAS TROUBLESHOT. THE LT PNEUMATIC BLEED AIR SHUTOFF VALVE WAS DETERMINED TO BE THE PROBLEM. THE VALVE IS DESIGNED TO FAIL IN THE OPEN CONDITION, IT REQUIRES POWER TO CLOSE IT. HOWEVER, DURING TROUBLESHOOTING IT WAS NOTED THAT THE VALVE HAD FAILED IN THE OPEN CONDITION, BUT WHEN ANY SORT OF BACK PRESSURE WAS APPLIED TO THE OUTLET OF THE VALVE IT WOULD CLOSE, WITHOUT POWER APPLIED. THE VALVE WAS REPLACED WITH AN O/H UNIT AND OPERATION RETURNED TO NORMAL.