DURING LANDING ROLLOUT THE PILOT LOST LEFT BRAKE PRESSURE AND THE AIRCRAFT GROUND LOOPED. POST CRASH EXAMINATION REVEALED THAT THE 'B-NUT' ON THE LEFT BRAKE LINE, WHERE IT PASSES THROUGH THE FUSELAGE, WAS LOOSE. ADDITIONALLY, HYDRAULIC FLUID WAS OBSERVED INSIDE THE FUSELAGE IN THE VICINITY OF THE FITTING, AND THE LEFT BRAKE MASTER CYLINDER CONTAINED ONLY A SMALL AMOUNT OF FLUID. THE PILOT REPORTED THAT HE WAS ASSISTING A MECHANIC IN MAINTENANCE ON THE LEFT LANDING GEAR SUPPORT BRACKET AND COULD NOT REMEMBER TIGHTENING THE 'B-NUT' OR IF THE MECHANIC INSPECTED THE WORK HE ACCOMPLISHED.