The pilot landed on runway 18L. During the landing rollout, the airplane “swerved” to the right with application of brakes. The pilot corrected back to the runway centerline with rudder. He applied brakes again and the airplane again “swerved” to the right. He again corrected back to the runway centerline and applied the brakes a third time. The airplane again “swerved” to the right. He then applied the emergency brake. The airplane departed the right side of the runway and struck the concrete pad of a runway marker sign. An FAA inspector reported finding hydraulic fluid covering the left main landing gear. He said the main and auxiliary left brake lines were both fractured, and the right main landing gear tire was “shredded off the wheel.” According to NTSB’s Materials Laboratory Factual Report, the brake line transmits hydraulic fluid and the auxiliary brake line transmits compressed nitrogen. Both brake line fractures were the result of fatigue cracking. No mechanical or other damage was evident on the exterior of the line in the initiation area. Maintenance records dating back to the airplane’s date of manufacture revealed the brake lines had never been removed or replaced. Probable Cause: Failure of both the left main and auxiliary brake lines due to fatigue cracking, rendering directional control impossible. A contributing factor in this accident was the airport sign concrete pad.