What was found
AFTER RETURNING THE PILOT REPORTED A "SPONGY" LEFT BRAKE AND REDUCED BRAKING DURING TAXI. THE MASTER CYLINDER WAS DISASSEMBLED. THE FLUID OUT OF THE CYLINDER WAS BLACK AND APPEARED TO HAVE BLACK O-RING MATERIAL CONTAMINATING THE FLUID. THE MASTER CYLINDER WAS CLEANED, INSPECTED, AND REPACKED WITH NEW O-RINGS. THE SYSTEM WAS FLUSHED, BLED, AND OPS CHECKED NORMAL. THE O-RINGS APPEARED VERY WORN.