What was found
(NI6R) ON APPROACH FOR LANDING, THE AIRCREW DISCOVERED THE MAIN HYD SYS WOULD NOT BUILD PRESSURE ABOVE 1,000 PSI. NORMAL IN-FLIGHT OPERATING RANGE IS 1,400 - 1,725 PSI. AN IN-FLIGHT EMERGENCY WAS DECLARED, THE ACFT LANDED SAFELY, AND WAS TOWED FROM THE ACTIVE RUNWAY. DISCREPANCY WAS SUBSEQUENTLY DUPLICATED BY MX AND TROUBLESHOOTING REVEALED THE HYD PRESSURE REGULATOR VALVE WAS CHATTERING AND BYPASSING FLUID INTERNALLY. THE VALVE WAS R & R, PROPERLY ADJUSTED, AND FUNCTIONALLY CHECKED GOOD. DISASSEMBLY OF THE VALVE REVEALED INTERNAL WALL TO BE WORN AND DAMAGED.