What was found
ON CLIMBOUT AT 20,000 NOTICED LT DUCT PRESSURE 18-20 PSI LESS THAN RT. STAYED THAT WAY UNTIL LEVEL OFF AT 35,000 WHEN PRESSURE MATCHED. MATCHED THROUGHOUT CRUISE. LATER IN FLIGHT DURING CLIMB FROM 35,000 TO 37,000 HAD IDENTICAL SPLIT. PERFORMED T/C NG-36WC-00001 ENGINE BLEED AIR HEALTH CHECK. PRSOV AND PRECOOLER CONTROL VALVE FAILED. REPLACED BOTH VALVES, LEAK & OPS CHECKED OK IAW MM 36-12-02-400-801, MM 36-11-04-400-801. PERFORMED REPAIR COMFIRMATION FIM TASK 36-10-00-810-807, PARA G. COMFIRMATION CHECKED OK.