What was found
ON CLIMB OUT FROM MKE EN ROUTE FOR PHX, STANDBY PRESSURIZATION FAILED AT 35,000 FT, TURNED BACK AND LANDED AT MKE WITHOUT FURTHER EVENT. MAINTENANCE REMOVED AND REPLACED THE CABIN PRESSURE CONTROLLER IAW BMM 21-31-21, AND CABIN PRESSURE CONTROL PANEL IAW BMM 21-31-25. CABIN PRESSURIZATION INTEGRITY CHECK, AND CABIN PRESSURE OPERATIONAL CHECK WAS GOOD FOR BOTH UNITS.