What was found
PIT - FL 88 - AFTER TAKEOFF, LANDING GEAR WOULD NOT RETRACT. LEFT SIDE HYDRAULIC QUANTITY AND PRESSURE WENT TO ZERO. SYSTEM PRESSURE LIGHT ILLUMINATED AND CREW RECEIVED EICAS MESSAGE 'L HYD SYS PRESS'. TOWER ADVISED THAT SMOKE WAS COMING FROM THE LEFT SIDE OF THE AIRCRAFT. FLIGHT RETURNED TO PIT AND LANDED WITH NO FURTHER INCIDENT. MAINTENANCE REPLACED THE LEFT MAIN LANDING GEAR DOWNLOCK LINE. (M)