According to the flight instructor in the multi-engine airplane, he occupied the right seat during the instructional flight. The flight instructor reported that during the landing roll, the brakes on his side were inoperative, but the brakes used by the pilot under instruction appeared to function normally. The flight instructor allowed the pilot under instruction to taxi to parking. During the right turn in the parking area, the pilot under instruction was told to apply both brakes, and the airplane started to "pull left". The flight instructor attempted to use differential thrust to arrest the left pull by increasing the right engine speed, but the attempt had no effect. The flight instructor shut down both engines, however, the multi-engine airplane's left wing struck a parked single-engine airplane and came to a stop. The multi-engine airplane sustained substantial damage to the left wing. The flight instructor reported that the accident could have been prevented if he would have stopped on the taxiway and not entered the parking ramp. The brake assembly was examined to determine the scope of pre-accident mechanical malfunctions or failures with the airplane's brake system, that would have precluded normal operation. It was determined that the right side brake actuator piston "o" ring failed, which decreased hydraulic brake pressure, preventing proper brake function of the airplane.