The aircraft was landing downwind on an agricultural strip. During the landing roll the aircraft departed the left side of the strip following the failure of the right brake coming to rest inverted in a rice paddy. The investigation found the right brake master cylinder had buckled in the reservoir area. The buckling and resultant misalignment of the piston rod caused a bending failure of the rod and liberation of the piston with a subsequent loss of hydraulic pressure to the right brake. The reason for the failure of the master cylinder was not determined. There are no other recorded accidents involving master cylinder failures in this type of aircraft. The failure is therefore considered an isolated case.