What was found
(CAN) DURING ENGINE NR 2 START, THE GROUND AGENT REQUESTED THE CREW TO IMMEDIATELY SHUTDOWN THE ENGINE. CREW NOTICED SOME SMOKE IN THE COCKPIT BUT NO OTHER INDICATION. CREW DECIDED TO PROCEED TO AN GROUND EMERGENCY EVACUATION INCLUDING THE TRIGGERING OFF 1 FIRE EXTINGUISHER BOTTLE. GROUND CREW INFORMED SMOKE WAS COMING FROM NR 3 MAIN WHEEL. HYDRAULIC POWER APPLIED AND BRAKES OPERATED, FOUND LEAK FROM A BLANKING PLUG ON NR 4 BRAKE UNIT. O-RING REPLACED ON FWD BLANKING PLUG NR 4 BRAKE. ACFT RTS.