What was found
POWER BRAKE VALVE REPLACED FOR MAKING UNUSUAL NOISES DURING BRAKING. ON TAXI OUT, COPILOT'S RT BRAKE PEDAL LOST BRAKING ACTION. AIRCRAFT RETURNED TO GATE, MAINTENACE TROUBLESHOOTING FOUND BRAKE ACCUMULATOR TO BE AT FAULT, REINSTALLED ORIGINAL POWER BRAKE VALVE, REPACED ACCUMULATOR AND BLED BRAKES, OPS CHECK GOOD. AIRCRAFT RETURNED TO SERVICE.