What was found
T/O WARNING TEST ACCOMPLISHED ON TAXI OUT WAS OK. WHILE HOLDING SHORT OF THE RUNWAY WITH PARK BRAKE ON, CLEARED FOR TAKEOFF PARK BRAKE WAS RELEASED WHILE ROLLING ONTO THE RUNWAY, PARK BRAKE CAUTION, ILLUMINATED THEN WHEN OUT. TAKEOFF WARNING TEST ACCOMPLISHED AND IT FAILED. R & R PARKING BRAKE CONTROL QUADRANT, REF AMM TASK 32-44-21-000-801, 32-44-21-400-801. FUNCTIONAL TEST GOOD. C/W OPS TEST OF THE TAKE OFF WARNING SYSTEM REF AMM TASK 31-52-00-710-801. OPS TEST GOOD, NO OTHER DEFECT NOTED.