What was found
CAB PRESS REG 1 FAIL W/#2 ON MEL @ 8000' MAN PRESS SELECTED - VERY SLOW TO RESPOND RTB UPON ENTERING THE COCKPIT, FOUND #1 REGULATOR FAULTED. RESET #1 REGULATOR WOULD QUICKLY FAULT AFTER 30 SECONDS. PER CREW DEBRIEF, FWD OUTFLOW VALVE WAS NEAR CLOSED AND AFT OUTFLOW WAS NEAR OPEN WHEN SELECED MANUAL CONTROL AT 8000'. 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, #1 PRESSURE CONTROLLER DID NOT FAULT, PASSED ITS "OK" TEST AND PASSED ITS BITE TEST. IAW TSM 21-30.