The pilot flew his airplane to a mechanic at a fixed base operator (FBO) for repairs to the airplane's brakes. The pilot reported that next day he called the mechanic and the mechanic indicated that the repairs would be complete and the airplane could be picked up later in the day. He told the mechanic that he would pick up the plane that evening. The pilot said that he called the FBO where the person at the front desk found his keys and indicated that they would be waiting for him at the front desk. There was no note on the keys indicating that the repairs were not complete. The pilot preflighted the airplane and the brakes and wheels appeared normal. The brakes were "firm" when pressure was applied. The airplane was started and the pilot proceeded to taxi the airplane. He stated that the airplane did not stop when pressure was applied to both brakes. The airplane sustained substantial damage when it impacted a standing fuel truck. The pilot said that the mechanic left him a voice mail message indicating the brakes had not been repaired and that parts would be in the next day. According to the pilot, he was not aware of the mechanic's message until after the accident.