What was found
LEFT ENG BLEED OFF LIGHT ON & LT ENG BLEED OFF EICAS MESSAGE, WITH ENG RUNNING, ENG BLEED SWITCH ON, LT & RT ISOLATION VALVES CLOSED, AC PACK SELECTED ON, & DUCT PSI AT ZERO. INTERMITTENT ON FINAL APPROACH. SENSE LINES CHECKED, VALVES EXERCISED AND INTERROGATED AIR SUPPLY BITE BOX. SUSPECT PRSOV THE PROBLEM. FOUND FITTING FOR SUPPLY PRESSURE LINE AT PRESSURE REGULATING VALVE STRIPPED AND LOOSE. REPLACED VALVE FITTING AND GASKET. RAN ENGINES BLEED PRESSURE'S CHECK GOOD.