What was found
DURING CRUISE FLT AT FL350, AUTO PRESSURE FAIL LIGHT ILLUMINATED. SWITCHED PRESSURE MODE CONTROL TO ALTN & LIGHTS REMAINED ON & DIFF PRESSURE BEGAN TO RISE. DIFF PRESSURE REACHED 8.7 PSI, PRESSURE MODE CONTROL SWITCHED TO MANUAL & SLIGHTLY OPENED OUTFLOW VALVE DIFF PRESSURE BEGAN TO DESCEND & CABIN ALT TO RISE. ATTEMPTED TO CLOSE OUTFLOW VALVE, VALVE WOULD NOT CLOSE FULLY, CABIN ALT BEGAN TO RISE. ATC CONTACTED FOR EMERGENCY DESCEND & LANDED W/OUT INCIDENT. INSP OF CURRENT ACRFT ACTIVITY & PAST FAULT REPORTS BY ENGINEERS INDICATED A REAR GALLEY WATER LINE LEAKED. R/R`D REAR GALLEY WATER LINE & OUTFLOW VALVE. GROUND AND FLT TESTS WERE ACCOMP SUCCESSFULLY & A/C DETERMINED READY FOR RETURN TO SERVICE.