On August 31, 1995, at 0800 mountain daylight time, an Aerostar PA-60-601P, N8072J, collided with vehicles while taxiing at Jefferson County Airport, Broomfield, Colorado. The private pilot and passenger were not injured and the aircraft sustained substantial damage. Visual meteorological conditions prevailed for the Title 14 CFR Part 91 business flight and no flight plan was on file. The flight was preparing for departure to Billings, Montana. According to the pilot, while he was taxiing for takeoff, the left brake failed. He was attempting to taxi back to maintenance facilities using differential power for directional control when he lost control of the aircraft and struck two vehicles parked beside the ramp area. Examination of the aircraft by this investigator and a Federal Aviation Administration inspector provided evidence that the left brake was leaking at the "O" ring seals. The right brake was firm and functional and the nose wheel steering was functional. According to the Manager of Windsong Aviation, who maintains the aircraft, with the right brake and nose wheel steering functional, the aircraft was fully controllable.