What was found
DUCT PRESSURE SPLIT L=44 PSI R= 26 PSI AT CRUISE 35000FT N1 = 89.1 N2= 93.2 AFTER 4 HOURS OF FLIGHT THE PRESSURE BUMP LEFT CABIN RATE DESCENDED 400FPM AND DUCT PRESSURE SPLIT CHANGE TO L=40 PSI AND R= 32 PSI THAN BOTH TO ORIGINAL SPLIT 44/26. PERFORMED ENGINE BLEED AIR SYSTEM HEALTH CHECK ON NR 2 ENG REF MM 36 11 00 700 801 ENG FAILS THE REVERSE FLOW CHECK PROTECTION TEST STEP I. REPLACED THE HIGH STAGE REGULATOR REF MM 36 11 07. RE-PERFORMED THE TEST AND TEST NOW PASSES. OPS NORMAL.