What was found
THE FLIGHT RETURNED TO DFW FOLLOWING THE "L BLEED" LIGHT ILLUMINATING TRIPPING PACK AND BLEED OFF. WITH ENGINE AND WING ANTI-ICE ON, AIRCRAFT WILL NOT HOLD CABIN ALTITUDE. CORRECTIVE ACTION: MAINTENANCE REPLACED THE LT PACK CONDENSER AND WATER EXTRACTOR PACKING. CLEANED LT & RT PRE-COOLERS. REPLACED LT & RT NORMAL TEMP SENSORS. PERFORMED LT & RT PNEUMATIC LEAK CHECKS. REPLACED LT & RT HIGH STAGE PILOT VALVES & LT HPSOV. PERFORMED SINGLE PACK CONFIDENCE CHECK & ENGINE RUN PNEUMATIC LEAK CHECK. ALL PARAMETERS NORMAL EXCEPT LT PRE-COOL TEMP WITH ANTI-ICE ON. LOCKED FAV OPEN AND REPEATED RUN WITH NO CHANGE. SUSPECT PRE-COOLER. LOCKED OUT LT HPSOV PER MEL 36-11-02 AND TEMPS NORMAL. OK FOR SERVICE.