According to the flight instructor, during the taxi to the runway, the pilot receiving instruction stated that he did not have authority in the right brake. The flight instructor reported that he too applied pressure to the right brake to no avail. He reported that the airplane exited the runway to the left, impacted a drainage culvert and the left main landing gear collapsed. The airplane sustained substantial damage to the left wing spar. The airplane brake system was examined by the operator's Director of Maintenance, who determined that the right brake master cylinder was low on fluid causing the brakes to be weak. The last maintenance performed on the brakes occurred on 7/8/2015 at which time both brake discs were replaced and all the brake pads were replaced. The total time on the airplane since the last maintenance on the brakes was 33.6 hours.