What was found
DUCT PRESSURE DIFF OF 21PSI NOTICED A 37,000FT. LT IS 19 PSI, RT IS 40 PSI. PRESSURIZATION REMAINS NORMAL. ACFT GROUNDED. ACCOMPLISHED BLEED AIR HEALTH CHECK T/C NG-36WC-00001 ON NR 1 ENGINE, FOUND THE PRECOOLER CONTROL VALVE WOULD NOT OPEN OR CLOSE PROPERLY. REPLACED PRECOOLER CONTROL VALVE REF AMM 36-12-02. FOUND HIGH STAGE VALVE LEAKS INTERNALLY. REPLACED HIGH STAGE VALVE. OPS & LEAK CHECKS NORMAL. REQUIRES BLEED AIR CROSSOVER OPS TEST. ACCOMPLISHED ENGINE BLEED AIR CROSSOVER OPS CHECK IAW AMM 36-11-00-710-801. OPS CHECK GOOD. NO DEFECTS NOTED AT THIS TIME.