The pilot reported that he had been concerned about the performance of the aircraft's brakes and, following the first flight for the day, he had checked the pads for wear. They appeared to be serviceable with as much as 10 mm of pad material remaining. During the landing roll following the second flight, the aircraft yawed to the left. Corrective action using rudder and brake had no affect and aircraft continued to yaw to the left until it departed the 15 m-wide flight strip and collided with a fence and tree. A technical investigation disclosed that the brake system was fitted with spacing washers designed to prevent the brake pads being worn down to their backing plates. This prevented damage to the brake discs. The spacing washers were set so that around 10 mm of the pads remained when the brakes reached their maximum wear limit. The operator and pilot were not aware of this limitation and assumed from the amount of pad material remaining that the brakes were still serviceable. When the pilot applied the brakes to prevent the aircraft yaw, movement was restricted by the spacing washers and he was unable to obtain effective braking.