What was found
(CAN) CREW NOTICED A DECREASE IN CABIN DIFF. WHEN ACFT RETURNED TO MX BASE. IT WAS GROUND RUN AND THE LT FLOW CONTROL UNIT WAS NOT SUPPLYING THE REQUIRED AIR FLOW TO MAINTAIN THE CABIN DIFFERENTIAL. FLOW CONTROL UNIT WAS REPLACED WITH A SERVICEABLE UNIT AND GROUND TESTED. THIS UNIT WAS INSTALLED ON THE ACFT OCTOBER 21/09 AND ONLY HAD 163.5 HOURS ON IT. THIS IS THE 3 O/H UNIT INSTALLED THAT HAS NOT PERFORMED. 1FAILED ON INSTALL, THE 2ND OPERATED FOR 10 HOURS, BOTH OF THESE UNITS HAVE BEEN SENT BACK TO THE O/H FACILITY FOR EVALUATION.