As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING TAXI OUT, CAPTAIN'S LT BRAKE PEDAL WENT ALL THE WAY TO STOP WHILE ATTEMPTING BRAKE APPLICATION, WITHOUT BRAKE EFFECTIVENESS. MADE 2 ATTEMPTS W/O SUCCESS. CO-PILOT BRAKES WORKED AND STOPPED ACFT. CAPT SIDE BRAKES REGAINED EFFECTIVENESS SHORTLY THEREAFTER. RETURNED TO FBO. FOUND PILOTS LT MASTER BRAKE CYL DEFECTIVE. REMOVED DEFECTIVE CYL. INSTALLED O/H CYL. OPS & LEAK CK GOOD, WORK DONE IAW MM CHAPTER 32-40-00.