What was found
FLT 693 - CLT-PHL - DURING CLIMB FLOW LIGHT ILLUMINATED. LEFT AIRCRAFT SUPPLY PRESSURE 0 PSI. RIGHT SUPPLY 30 PSI. OUTFLOW VALVE MANUALLY CLOSED DID NOT HELP. ON DESCENT LEFT PRESSURE INCREASED. CABIN WAS NEVER ABOVE 8000 FEET. MASKS DID NOT DEPLOY. FLIGHT RETURNED TO CLT FOR MAINTENANCE. MAINTENANCE REPLACED LEFT PILOT PRESSURE REGULATOR VALVE. OPS AND LEAK CHECKS GOOD. (M)