What was found
ON PUSH BACK (SAP), BRAKES DID NOT RELEASE. UNABLE TO MOVE AIRCRAFT UNDER NORMAL POWER. RETURNED TO BLOCK. AFTER TROUBLESHOOTING AND SYSTEM TEST FOUND BAD DUAL PWR.BRAKE VALVE. REPLACED VALVE. REPLACED AS PRECAUTIONARY MEANS BOTH THE LT MLG SHUTTLE VALVE, AND LT BRAKE CONTROL VALVE. PREFORMED SYSTEM FUNCTIONAL TEST, ANTI SKID FUNCTIONAL TEST, AIRCRAFT TAXIED WITH NO FURTHER INCIDENTS. AIRCRAFT RETURNED TO SERVICE.