The pilot reported that during engine start up, he applied the brakes to the cargo airplane and remained stationary while he received a taxi clearance. As he released the brakes to begin taxiing, the brake pedals went to the floor with no braking action. He pumped the brakes several times with no effect, so he feathered both propellers and shut down the engines. The airplane continued to roll and collided with a parked airplane on the ramp. Postaccident examination revealed that the airplane sustained substantial damage to the left wing. A functional check revealed that the brake system was operational with no malfunctions noted. The left main landing gear brake pads were worn beyond allowable limits. The examination did not reveal any other anomalies that would have precluded normal operation. While the left brake pads were worn beyond allowable limits, the main landing gear braking action would have still been available to the pilot.