What was found
BWI - FLT 1245 - AFTER TAKEOFF, CREW COULD NOT GET AIRCRAFT TO PRESSURIZE. AIRCRAFT WAS HELD AT 9,000 FEET WHILE CONVERSING WITH MAINTENANCE. AFTER SEVERAL FAILED ATTEMPTS, CREW DECIDED TO RETURN TO FIELD AT BWI. NO EMERGENCY WAS DECLARED AND THE FLIGHT LANDED WITH NO FURTHER INCIDENT. MAINTENANCE REPLACED THE RIGHT AIR CYCLE MACHINE, THE LEFT 35 DEGREE WATER SEPARATOR SENSOR AND THE LEFT AND RIGHT WATER SEPARATOR BAGS. OPS CHECK GOOD. PRESSURIZATION CHECK OF AIRCRAFT GOOD PER MAINTENANCE MANUAL. REFERENCE USAASB96163.