What was found
LEFT BLEED TRIP OFF LIGHT AT CRUISE, FL 380, ENGINE AND WING AN TI-ICE WERE ON. TURNED OFF WING ANTI-ICE RESET L BLEED TRIP LIGHT. LEFT DUCT WAS ZERO WITH LIGHT ON. WHEN RESET WAS PUSHED, LIGHT EXTINGUISHED, LEFT DUCT PRESSURE NORMAL-TURNED WING ANTI- ICE ON AGAIN AND LEFT BLEED TRIP LIGHT CAME ON AGAIN DUCT PRESS LEFTSIDE TO ZERO TURNED OFF WING ANTI-ICE & RESET BLEED TRIP LT OFF PERFORM NR 1 ENG BLEED AIR SYSTEM HEALTH CHECK PER MM 36-11-00 AND FIM 36-10-00-810-801. REMOVED AND REPLACED NR 1 ENG PRECOOLER CONTROL VALVE PER MM 36-12-02. OPS CHECKS GOOD DURING ENG RUN. NO BLEED TRIP NOTED. OK FOR SERVICE. ROTABLE PARTS CHANGE WAS INDICATED - TT # 1245906 SN OFF 03031 VALVE, PRECOOLER OFF 03-3617-9-0001 3031 ON 29-3617-9-0001 4077.