On November 13, 2004, at 1430 Pacific standard time, a Great Lakes 2T-1A, N502GL, ground looped on the landing rollout on runway 28L at Montgomery Field Airport (MYF), San Diego, California. Associated Aircraft, Inc., operated the airplane as an instructional flight under the provisions of 14 CFR Part 91. The airplane sustained substantial damage. The certified flight instructor (CFI) and student pilot were not injured. Visual meteorological conditions prevailed for the local area instructional flight that departed MYF at 0900. A stop was made at Borrego Valley Airport (L08), Borrego Springs, California. The flight departed L08 for Montgomery at 1335. The flight was destined for Montgomery Field. According to the CFI, the purpose of the flight was to provide a conventional gear airplane checkout for the private pilot. They encountered no discrepancies with the flight until the landing rollout at MYF. The CFI reported that on approach he took the flight controls to demonstrate a forward slip to lose altitude. He indicated that this was a normal approach as the airplane is not equipped with flaps. He made a normal three-point landing, and as the airplane was rolling out straight with the control stick full back, he attempted to slow down with light pressure on both heel brakes. The airplane started a right turn, and he counteracted with the left brake; however, there was no response. The airplane swerved off the runway and ground looped to the right. The lower left wing contacted the ground, and the airplane came to rest on its nose. A Federal Aviation Administration (FAA) inspector examined the airplane's brake system and found that the brakes were a little "spongy" when he depressed them, but functioned normally. There were no further mechanical discrepancies noted.