What was found
DURING TAXIING THE F/O'S BRAKES FELT SOFT, CAPTAIN'S SIDE WAS GOOD. AFTER TAKEOFF CONTINUED TO OBSERVE BRAKE PRESSURE, ALL NORMAL. AT CRUISE NOTICED A VIBRATION AND HUMMING SOUND, DIVERTED AIRCRAFT INTO MEM AND DECLARED EMERGENCY FOR LANDING. BRAKES OPS CHECK GOOD ON GROUND, JACKED AIRCRAFT FOUND BRAKE PUMP CYCLED PROPERLY. SWUNG GEAR AND PUMP STOPPED, PRESSURE BLEEDS OFF, WHILE BRAKE WAS BEING BLEED LOUD BANG FROM POWER BRAKE VALVE. REPLACED BRAKE VALVE, LEAK CHECKED AND OPS CHECKED GOOD, ALL WORK DONE IAW CESSNA 560 MM CHAPTER 32-42-05.