A ART 91 SMT ROLLED FORWARD AFTER ENGINE START AND HIT A PARKED ACFT. MINOR DAMAGE TO BOTH ACFT.

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THE PLT STARTED ENGS WITH FEET ON BRAKES AND PARKING BRAKE ON. PLT CHKED ENG GAUGES AND ITEMS ON CHKLIST. PLT SPENT APPROX 15 SECS LOOKING AT ITEMS IN THE AIRPLANE. THE LAST ITEM ON CHKLIST WAS THE REACHING TO THE FLOOR BTWN THE SEATS TO PICK UP FLT LOG TO WRITE DOWN START TIME AND OTHER DATA. AS SOON AS THE LOG REACHED THE PLT'S LAP; THE PLT FELT THE ACFT MOVING AND THE BRAKES WERE NO LONGER SNUG TIGHT TO THE FLOOR. THE PLT LOOKED UP; SAW THE OTHER AIRPLANE 10' AHEAD; SHUT DOWN MIXTURES ON BOTH ENG AND PRESSED BRAKES HARDER. THE SMT STOPPED WITH THE LEADING EDGE OF RIGHT WING HITTING RUDDER TAIL BOTTOM OF THE SMA. MINOR DAMAGE TO BOTH ACFT. NO ONE INJURED. THE CAUSES ARE POSSIBLY ONE OF THESE: PLT NOT HOLDING BRAKES TIGHT ENOUGH AFTER ENG START; PARKING BRAKE RELEASED PRESSURE ALLOWING PLANE TO MOVE FORWARD; OR BRAKES HAD DIRT IN PLUNGER AREA ALLOWING MOMENTARY RELEASE OF PRESSURE. PREVENTION OF OCCURRENCE: PARK UPHILL WITH NOTHING IN FRONT OF ACFT; 2-PLT CREW; BETTER LIGHTING OF PARKING AREA AND PARK UPHILL.