AN SMT ROLLED ONTO THE OVERRUN AT THE END OF THE RWY.

Narrative

I WAS FAST TAXIING IN AN SMT ON RWY 30 AT SAN CARLOS ARPT WITH THE MECH. THE CTL TWR HAD PREVIOUSLY GIVEN AUTHORIZATION FOR THE FAST TAXI IN ORDER TO 'BURN-IN' THE BRAKES WHICH HAD JUST BEEN CHANGED. 3 HIGH SPD RUNS HAD BEEN COMPLETED WITH SPDS FROM 0-50 MPH AND THE BRAKES APPLIED. ON THE THIRD RUN; SPD WAS INCREASED TO 50 MPH AND BRAKING APPLIED WITH ABOUT 1000 FT OF RWY USED BEFORE APPLYING BRAKES. THE BRAKES ACTED NORMALLY AND APPEARED TO BE BURNING IN PROPERLY. ON THE FOURTH RUN THE SAME PROC; HOWEVER; THE BRAKES DID NOT STOP THE WHEEL ROTATION. THE ACFT CONTINUED ON THE RWY DECAYING SPD BUT NOT COMPLETELY STOPPING THE ACFT PRIOR TO REACHING THE END OF THE RWY. THE ACFT CONTINUED PAST THE END OF THE RWY ONTO THE OVERRUN; DECAYING THE SPD FURTHER. THE EXACT SPD WAS NOT DETERMINED. A 180 DEG TURN WAS MADE JUST PAST THE END OF THE RWY ON THE OVERRUN. THE TWR AUTHORIZED 2 MORE HIGH SPD TAXI RUNS AND ALSO REQUESTED I RPT THE EVENT TO THE ARPT OFFICE AFTER COMPLETION OF TAXI. 2 MORE HIGH SPD TAXI RUNS WERE MADE AND THE BRAKES APPEARED TO BE SATISFACTORY. THE RPT WAS MADE TO THE ARPT OFFICE AND THE ATTENDANT THERE SAID THERE WAS A TECHNICAL VIOLATION OF THE FARS DUE TO THE ACFT BEING ON THE OVERRUN; EVEN IF INADVERTENT. THERE WAS NO DAMAGE TO THE ACFT; RWY OR OVERRUN.