What was found
CABIN PRESSURE REGULATOR NR 1 FAILED WITH THE NR 2 ON MEL AT 8000 FT MANUAL PRESSURE SELECTED, VERY SLOW TO RESPOND, AIR RETURN. ON ENTERING THE COCKPIT, FOUND THE NR 1 REGULATOR FAULTED. RESET THE REGULATOR, WOULD QUICKLY FAULT AFTER 30 SECONDS. FWD OUTFLOW VALVE WAS NEAR CLOSED AND AFT OUTFLOW WAS NEAR OPEN WHEN SELECED MANUAL CONTROL AT 8000T. RAN BOTH OUTFLOW VALVES MANUALLY FULL OPEN TO FULL CLOSED SEVERAL TIMES. FWD VALVE SMOOTH AND RESPONDED TO COMMANDS IN MANUAL AND AUTO. AFT OUTFLOW VALVE WAS ERRATIC IN MANUAL AND FAILED IN AUTO. WHEN VALVES DRIVEN TO FULL OPEN AND HELD FOR 30 SECONDS, THE NR 1 PRESSURE CONTROLLER DID NOT FAULT, PASSED, OK TEST AND PASSED THE BITE TEST IAW TSM 21-30.