What was found
ON DEPARTURE FROM SEATTLE THE LEFT DUCT PRESSURE INDICATED 80 PSI AND RIGHT DUCT PRESSURE INDICATED 42 PSI. CONSULTED WITH FODO AND MAINT CONTROL VIA SAT BECAUSE AT LIMITATION EXCEEDANCE. DURING CLIMBOUT TO FLT 370 LEFT DUCT STARTED TO DROP AND ONCE LEVELING AT FL370 DUCT PRESSURE (LEFT AND RIGHT) STABILIZED AT 39 PSI. GROUNDED - YES. OPENED T/R REF B737-800 AMM 78-31-00. DURING GVI INSPECTION OF PRSOV FOUND PRSOV LEAKING FROM MISSING PLUG. INSTALLED NEW PRSOV PLUG. PERFORMED OPS/LEACK CHECK, REF AMM 36-11-04. NO DEFECTS NOTED. REF FIM 36-10 TASK 807 STEP F(5-12) NO DUCT SPLIT NOTED GROUND VERIFICATION SUCCESSFUL.