What was found
IAW ACARS RECEIVED FROM FLT 480 INBD SAN AND ALSO CONVERSATION W/ PILOT AFTER LANDING, DUCT SPLIT INDICATED L 23 PSI R 42 PSI AT FL370. GAP NARROWED AS FLIGHT PROGRESSED TO L 28 PSI R 40 PSI. NO OTHER PROBLEMS NOTED. DUCT SPLIT WITHIN LIMITS. PRECAUTIONARILY SCHEDULED BLEED AIR SYS CHECK. ACCOMPLISHED BLEED AIR SYS HEALTH CHECK ON NR 1 ENGINE REF T/C NG-36WC-0001 FOUND PRECOOLER VALVE AND HIGH STAGE VALVE BAD. NO OTHER DEFECTS NOTED. R & R AND OPS CHECKED NORMAL NR 1 ENGINE PRECOOLER CONTROL VALVE REF AMM 36-12-02 AND R & R NR 1 HIGH STAGE VALVE. LEAK AND OPS CHECKED GOOD. REF AMM 36-11-06. ACCOMPLISHED ENGINE BLEED AIR CROSSOVER TEST AND OPS CHECKED GOOD REF AMM 36-11-00-710-801.