What was found
LEFT BRAKE STARTS OUT WORKING FINE, BUT FAILS WITH CONTINUED USAGE. BRAKE PEDAL GOES TO THE FLOOR WITHOUT STOPPING AIRCRAFT. INSPECTED RIGID HYDRAULIC LINE FROM MASTER CYLINDER TO ZONE 1 FOR LEAKS. REPLACED RIGID HYDRAULIC LINE FROM MASTER CYLINDER TO ZONE 1 IAW ATR42 M/M 20-23-13 BLED LOW PRESSURE LINE IAW ATR42 JIC 32-42 -00 BLD 10000 NO LEAKS FOUND TAXIED AIRCRAFT FOR EXTENDED AMOUNT OF TIME. BRAKES OPERATED CORRECTLY IAW ATR42 JIC 32-42-00. THE AIRCRAFT WAS RETURNED TO SERVICE.