SF34 CREW EXPERIENCES UNCOMMANDED PARKING BRAKE RELEASE AT PHL.

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WHILE PARKED AT PHL; AN UNCOMMANDED BRAKE RELEASE OCCURRED ON MY ACFT. WE HAD JUST COMPLETED THE AFTER START CHKLIST; WHICH WAS PRECEEDED BY SIGNAL FOR THE MARSHALLERS TO REMOVE THE NOSE GEAR CHOCKS; THEN PLACE THE CONDITION LEVERS IN THE 'MINIMUM' POS (THIS ACTION CHANGES THE PROP PITCH FROM FEATHER TO A POSITIVE BLADE ANGLE). MY FEET WERE ON THE PEDALS AND I CHKED OUTSIDE THE ACFT AS NORMALLY DONE. I THEN LOOKED DOWN FOR A MOMENT IN THE COCKPIT. I FELT A MOVEMENT AND LOOKED UP IMMEDIATELY AT THE CTL WARNING PANEL -- THE PARKING BRAKE LIGHT WAS ILLUMINATED; ALSO WITNESSED BY THE FO. WE SIMULTANEOUSLY APPLIED BRAKES AND THE ACFT STOPPED. I POINTED TO THE CTL WARNING PANEL LIGHT; HE ACKNOWLEDGED IT WAS ON. I POINTED TO THE BRAKE HANDLE; WHICH HE ALSO ACKNOWLEDGED -- THE HANDLE WAS IN THE PARKED POS. I TRIED MOVING IT TO RESET IT; IT COULD NOT BE MOVED; FURTHER INDICATING IT WAS SET. I TRIED TO EVALUATE IF I HAD PREVIOUSLY MISSET THE BRAKE OR IN FACT A MECHANICAL PROB EXISTED. NO INJURIES; NO DAMAGE. WE CONTINUED THE FLT TO DEST. THE PARKING BRAKE WAS SET MANY TIMES DURING THIS PORTION OF THE FLT. NO ABNORMALITIES.