What was found
DURING TAXI OUT, CO-PILOT'S BRAKES WENT TO FLOOR ON INITIAL APPLICATION, PILOT TOOK CONTROL OF BRAKES AND RETURNED TO FBO. MX CREW WENT TO AIRCRAFT, TROUBLESHOT, REPLACED CO-PILOT'S LT AND RT MASTER CYLINDERS, BLED BRAKED, OPERATION SATISFACTORY, RETURNED ACFT TO SERVICE.