What was found
DURING CLIMB FROM FL 350 TO FL 370, LEFT DUCT PRESSURE = 46 PSI, RIGHT = 23 PSI, EXCEEDS 20 PSI MAX. DURING CRUISE @ FL 370 RT = 34 LEFT = 43. GROUNDED - YES. FOUND PRSOV FAILED HEALTH CHECK, PRECOOLER CONTROL VALVE FAILED HEALTH CHECK AND HIGH STAGE VALVE FAILED HEALTH CHECK . REMOVED AND REPLACED ALL THREE VALVES PER B737-900 AMM 36-11-04 AND 36-12-02 AND 36-11-07. ALL LEAKS CHECKED GOOD & OPS CHECK GOOD. C/W ETOPS GROUND VERIFICATION PER MM 36-11-00-710-801 GROUND VERIFICATION - SUCCESSFUL.