What was found
LEFT BLEED TRIP OFF LT ILLUMINATED, ATTEMPTED RESET IAW QRH, LT WENT OUT FOR ABOUT 10 SEC, DUCT PRESSURE SPLIT L 22 PSI RT 45 PSI, RE-ILLUMINATED. ACFT GROUNDED. DIVERSION TIME - 23 MINUTES. PCV FAILS CONTROL PRESSURE REGULATION TEST, CONTROL PRESS READS OFF-SCALE ON 30 PSI GAUGE. REPLACED NR 1 ENG PRECOOLER CONT VALVE REF MM 36-12-02-400-801 & 000-801. OPS & LEAK CHECKS GOOD. ACCOMPLISHED BLEED AIR SYS HEALTH CHECK T/C NG-36WC-00001. HIGH STAGE REGULATOR FAILS SHUTOFF TEST. REPLACED HIGH STAGE REGULATOR REF MM 36-11-07-000-801 & 400-801. LEAK CHECK GOOD. ACCOMPLISHED LT BLEED CROSSOVER/TRIP CONFIRMATION REF MM 36-11-00-710-801, FIM 36-10-00-810-801.