What was found
CABIN PRESSURIZATION WOULD NOT HOLD 5.4 PSI AT FL250, WOULD NOT HOLD MAX DIFFERENTIAL. WITH ENGINE ANTI-ICE ON, WOULD NOT HOLD CABIN BELOW 10,000 FEET. PNEUMATIC PRESSURE GAUGE INDICATED 30 PSI IN CRUISE WITH PNEUMATIC CROSSFEEDS CLOSED. AFTER PRESSURE WAS BLEED OFF WITH AIRFOIL ANTI-ICE, THE 30 POUNDS OF PNEUMATIC PRESSURE RETURNED IN APPROXIMATELY 10 SECONDS. CREW DONNED THEIR OXYGEN MASKS DUE TO THE CONDITION. FOUND THE LEFT AIR CONDITIONING PRESSURE REGULATOR VALVE STUCK OPEN. REPLACED THE LEFT AIR CONDITIONING PRESSURE REGULATOR VALVE IAW DC-9 MM 21-31-01-2-0003.