What was found
DURING CLIMBOUT, THE PACT DUCT PRESSURE SHOWED A SPLIT OF RT=42, LT=8PSI UP TO APPROXIMATELY FL280 THEN SHOWED A SPLIT OAT TOC RT=42PSI, LT=32. ONCE STABLE AT CRUISE THEY SHOWED EQUAL AT 40 PSI. DUCT PRESSURES REMAINED EQUAL THROUGHOUT REST OF FLIGHT. ACFT GROUNDED. ACCOMPLISHED PNEUMATIC SYS T/S, REF AMM 36-11-00-700-804. FOUND HI-STAGE VALVE STUCK HALFWAY OPEN AND PRSOV LEAKING. R & R HSV AND PRSOV. GROUND VERIFICATION TESTS, OPS AND LEAK CHECKS OK, REF AMM 36-11-06/400.