On April 27, 2005, about 1930 Alaska daylight time, a tundra tire equipped Piper PA-18-160 airplane, N624MW, sustained substantial damage following a loss of control and collision with a ditch while landing at the Wasilla Airport, Wasilla, Alaska. The solo private pilot was not injured. The 14 CFR Part 91 local personal flight operated in visual meteorological conditions without a flight plan. The flight departed Wasilla about 1700. During a telephone conversation with the NTSB investigator-in-charge (IIC) on April 29, the pilot related that he was landing on the dirt airstrip, runway 21, at Wasilla, when the main landing gear tires encountered a rut, and he lost directional control of the airplane. He said the airplane veered immediately to the left, and the left rudder "bottomed out" making it difficult to apply full right rudder and make a correction. He stated that he applied full right brake, but that the brake was ineffective, and may have failed to operate properly. The airplane continued to the left, and encountered a ditch. The pilot indicated that the airport manager should be made aware of the rutted runway, before someone else has an accident. Following the conversation with the pilot, the IIC contacted the Wasilla Airport manager. The airport manager indicated that there are no dirt or gravel runways at the Wasilla Airport, and that runway 21 is hard surfaced asphalt. He noted that there is an area before the approach end of runway 21 that is dirt and gravel, but it is not designated as a portion of the runway, and it is not maintained as such. A review of the FAA Alaska Supplement and Airport Directory disclosed only one runway at the Wasilla Airport, runway 21-03, which is asphalt covered. The IIC contacted the aviation mechanic who repaired the airplane. The mechanic stated that the right brake assembly had a slow leak, which could have allowed the brake pressure to bleed down and the brake to be less effective. The mechanic indicated he rebuilt the brake assembly following the accident.