The pilot reported that during his landing roll, he attempted to slow the airplane as he exited the runway for the taxiway. As he applied brakes, the left brake lost pressure and the right brake maintained its pressure and operated normally. This anomaly caused the airplane to ground loop, which resulted in substantial damage to the left wing. Examination of the airplane by a Federal Aviation Administration inspector revealed the left brake master cylinder was void of fluid. Fluid was filled into the master cylinder and when pressure was applied to simulate braking, fluid began to seep from the master cylinder. The master cylinder was disassembled revealing a damaged O-ring seal. The O-ring was pinched and cut; therefore, unable to maintain hydraulic fluid within the master cylinder and resulted in the loss of braking. A review of maintenance records reveal that the braking system was serviced about a month prior, but no anomalies were noted during the service.