What was found
AFTER RETURNING TO DEPARTURE, THE PILOT LOST FULL BRAKING POWER ON THE LT BRAKE AND REPORTED A "SPONGY" CONDITION AND WAS UNABLE TO PUMP UP BRAKE PRESSURE. THE ACFT LEFT MASTER CYLINDER WAS REMOVED AND DISASSEMBLED FOR INSPECTION. THE BRAKE FLUID COMING OUT OF THE WAS CONTAMINATED WITH BLACK MATERIAL FROM THE O-RINGS. THE INTERIOR WAS INSPECTED AND FOUND UN-SCORED. THE MASTER CYLINDER WAS RE-PACKED WITH NEW O-RINGS, BRAKE SYS FLUSHED, BLED, AND OPS CHECKED NORMAL.