AN ASTRA 1125 CPR FLC HAS A NEAR RWY EXCURSION DURING A LNDG ON A WET RWY AT PDK; GA.

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ATIS RPTED VARIABLE WIND AT 6 KTS; LATER AT 090 DEGS/6 KTS AND LATER AT 060 DEGS/11 KTS. PDK WAS USING ILS RWY 20L. AFTER A NORMAL TOUCHDOWN; I DISCOVERED THE BRAKES WERE INEFFECTIVE. THERE WAS A LOT OF STANDING WATER ON THE RWY AND THE TAILWIND WASN'T RPTED UNTIL SHORTLY BEFORE LNDG. AFTER I NOTICED THE BRAKES WERE NOT WORKING WELL BECAUSE OF HYDROPLANING; I GAVE A FULL 100% OF AVAILABLE REVERSE THRUST. AFTER TOUCHDOWN WE ALWAYS POP OUT REVERSER AND ADD ABOUT 70% REVERSE; AS SOON AS I NOTICED HYDROPLANING I IMMEDIATELY ADDED MORE REVERSE. WE STOPPED 1 INCH FROM THE END OF THE RWY. EVERYTHING WAS FINE; BUT IT COULD HAVE LED TO A BAD SIT. WE TURNED OFF RWY; NO PROB. THE RWY SURFACE HAS SEVERAL DIPS THAT HOLD EXCESS WATER. THE SMALLEST AMOUNT OF RAIN IS VERY HAZARDOUS. SUPPLEMENTAL INFO FROM ACN 487169: LOW TIME CAPT FLEW ILS RWY 20L AT PDK. CAPT STATED HE WOULD MAKE A GREASER DUE TO WET RWY. CAPT FLEW GOOD ILS AND BROKE OUT ABOUT 500 FT AGL. HE STAYED HIGH UNTIL I TOLD HIM HE WAS TOO HIGH. HE THEN STARTED DOWN AND TOUCHDOWN ALMOST 1/2 WAY DOWN RWY. HE WENT INTO MAX REVERSE BUT NOT VERY HARD ON BRAKES. AT ABOUT THE LAST 1000 FT; I LITERALLY STOOD ON THE BRAKES AND WE STOPPED WITH THE NOSEWHEEL ABOUT 12-18 INCHES OFF THE RWY. FROM THE SIC SEAT I STAYED IN MAX REVERSE AND BACKED THE ACFT FULLY ONTO THE RWY AND HE TAXIED TO THE RAMP. CAPT GOT OUT OF ACFT; DISPOSED OF PAX AND WENT HOME. I EXAMINED ACFT AND COULD NOT FIND ANY DAMAGE. ATC HAD NO REMARKS.