What was found
WHILE OPERATING A CFAR 135 REGULATED FLIGHT LEG WITH A DEPARTURE FROM KMSY WITH INTENTIONS TO FLY TO KPHX, THE AIRCRAFT INCURRED A MECHANICAL ISSUE. DURING THE CLIMB ABOVE APPROXIMATELY 10,000’ THE CABIN RATE BEGAN TO DESCEND AT A RATE OF 1,500 FT/MIN WHEN THE RATE SHOULD HAVE BEEN CLIMBING AT 500 FT/MIN. NOTICING THE AIRCRAFT WAS NOT FUNCTION AS DESIRED WE DECIDED TO RETURN TO KMSY AND HAVE THE PASSENGERS DEPLANE AND UTILIZE ANOTHER AIRCRAFT FOR THIS PARTICULAR FLIGHT. AT THIS TIME I DID NOT DEEM IT WAS SAFE TO CONTINUE ACROSS THE COUNTRY AND TO HAVE TO RETURN LATER THAT EVENING" MECHANICAL ISSUE WAS OVER PRESSURIZATION.