According to the flight instructor in the tailwheel-equipped airplane, during the landing roll the airplane encountered a wind gust from right, the airplane began to "weathervane" and he applied left rudder and differential braking. He reported that he was flying the airplane from the right seat and that, "Rather than attempt to keep the aircraft on the runway I allowed the aircraft to depart the right side of the runway under control and in a straight line." The flight instructor reported that he continued to apply the brakes and the airplane crossed the safety area, rolled on to the parallel taxiway, ground looped to the right and struck a drainage culvert. The airplane sustained substantial damage to the rudder. The airplane and more specifically the brake master cylinders were examined at the repair facility by a Federal Aviation Administration certificated Airframe and Powerplant Mechanic. The mechanic reported that the factory installed brake system was all original, and that the, inspection of the brake master cylinders revealed them to be un-airworthy due to aged and defective O-rings. He reported that, if air is introduced into either the left or right wheel brake master cylinders located on the left side of the airplane, then the brake pedals would be ineffective on either side of the airplane.