A C550 FLC EXPERIENCES A RWY EXCURSION WHEN THE ANTI-SKID SYS FAILS AND THE ACFT GOES 3 FT OFF THE END OF RWY 20L AT PDK; GA.

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WE HAD A NORMAL FLT WITH AN ILS APCH RWY 20L TO ATLANTA-DEKALB-PEACHTREE (PDK) ARPT. WE BROKE OUT OF THE CLOUDS AT 250 FT AGL AND MADE A NORMAL TOUCHDOWN ON RWY 20L. THE TOUCHDOWN SPOT WAS NORMAL AND THE INITIAL STOPPING DISTANCE WAS NORMAL. THE TWR INSTRUCTED US TO MAKE A R TURN AT THE END WITHOUT DELAY. I ADDED A LITTLE PWR TO INCREASE OUR SPD TO APPROX 20 KTS. AT WHAT I JUDGED TO BE A NORMAL SLOW DOWN POINT TO MAKE THE TURN AT THE END WITH A WET RWY; I DECREASED PWR TO IDLE AND APPLIED BRAKES. AS I APPLIED BRAKES I NOTICED THE ANTI-SKID FAIL LIGHT ILLUMINATE THEN GO OUT. A VERY SHORT TIME LATER (3-5 SECONDS) WILE BRAKING WAS STILL APPLIED; THE TIRES STARTED TO SKID. I NOTICED THE ANTI-SKID FAIL LIGHT WAS ILLUMINATED SOLID. I ELECTED TO TRY AND KEEP THE ACFT STRAIGHT AHEAD RATHER THAN ATTEMPT A TURN. THE ACFT CAME TO A COMPLETE STOP WITH THE NOSE GEAR APPROX 3 FT OFF THE END OF THE RWY. I NOTIFIED THE TWR AND SHUT THE ENGS DOWN. THERE WAS NO INJURIES; NO DAMAGE TO THE ACFT AND NO DAMAGE TO THE ARPT. MAINT FOUND A FAULTY ANTI-SKID PUMP.