What was found
FLT 1001 - MKE-STL - SHORTLY AFTER TAKEOFF, THE CREW COULD NOT PRESSURIZE THE CABIN AND MADE AN AIR-TURN-BACK. THE CREW TOLD MAINTENANCE THEY COULD NOT LATCH THE FO DOCUMENT WINDOW. MAINTENANCE FIXED THE WINDOW AND RETURNED THE A/C TO SERVICE. SHORTLY AFTER THE SECOND TAKEOFF, THE CREW COULD NOT PRESSURIZE THE CABIN AND MADE AN AIR-TURN-BACK. MAINTENANCE INSPECTED THE SYSTEM AND FOUND A BAD CHECK VALVE AT THE ACM. MAINTENANCE REPLACED THE CHECK VALVE AND CHANGED THE RT PRSOV. MAINTENANCE OPS CHECKED THE SYSTEM AND RETURNED THE AIRCRAFT TO SERVICE. (M)