What was found
PILOT BEGAN TO TAXI FOR TAKEOFF AND REALIZED HIS RIGHT BRAKE WAS UNRESPONSIVE. PILOT RETURNED TO GATE. UPON INSPECTION, FOUND BRAKE CALIPER PISTON PACKING TORN CAUSING LOSS OF BRAKE FLUID. BRAKE CALIPER WAS REMOVED, DISASSEMBLED, CLEANED, REASSEMBLED WITH NEW PACKINGS, AND RE-INSTALLED ON AIRCRAFT. BLED BRAKES. OPS CHECK GOOD AND AIRCRAFT RETURNED TO SERVICE.