What was found
ON TAXI OUT, CO-PILOTS BRAKE FAILED. INITIALLY HARD BRAKES WITH NO BRAKING. RIGHT PEDAL WENT TO FLOOR, THEN WITH 3 PUMPS, HAD BRAKES, BUT A/C SHUTTERED TO A STOP WITH SEVERE RUMBLE NOTICED. GAINED ACCESS TO BRAKE MIXING VALVES. REMOVED FAULTY RT MIXING VALVE P/N C100169, S/N 1265A. INSTALLED NEW MIXING VALVE P/N C100169, S/N 1296A. BLED BRAKES NUMEROUS TIMES IAW HAWKER 400XP MM CHAPTER 32-40-00. REINSTALLED ALL PANELS AND PARTS REQUIRED TO BE REMOVED. REFERENCING 400XP MM CHAPTERS 06-50-00, 20-00-00 & 25-11-00. OPS/ LEAK CHECK C/W. BOTH CHECK GOOD.