MD80 CREW LOSES BRAKING AFTER LNDG ON WET RWY AT DFW.

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PERFORMED AN AUTOLAND TO RWY 18R AT DFW; RVR 4000 FT. DURING ROLLOUT; TWR ASKED US TO MAKE THE HIGH SPD. AS PNF; I APPLIED BRAKES BUT BELIEVED OUR SPD TO BE EXCESSIVE FOR TURNOFF; SO I CONTINUED TO END DURING WHICH TIME TWR ASKED US TO EXPEDITE TO END. I ADDED PWR TO COMPLY AND AS WE APCHED THE END THE BRAKING ACTION WAS NIL TO POOR AND WE SKIDDED TO SEVERAL FT SHORT OF THRESHOLD LIGHTS. IN REVIEWING THE EVENT IN MY MIND; I SHOULD HAVE USED THE AUTO-BRAKING SYS TO STOP; EVEN THOUGH THE RWY CONDITION WAS NOT RPTED. IT WAS WET. MOREOVER; IN APPLYING PWR TO EXPEDITE RWY EXIT; I INCREASED OUR GND SPD ON THE SLICKEST PORTION OF THE RWY; THE LAST 500-1000 FT. WE STOPPED; BUT JUST BARELY. SUPPLEMENTAL INFO FROM ACN 424730: PF USED MAX BRAKING EFFORT TO STOP JUST SHORT OF RWY END LIGHTS. ACFT SKIDDED LAST 100-200 FT PRIOR TO STOP. JUST ENOUGH ROOM TO HARD TURN OFF OF RWY. NO BRAKING ACTION HAD BEEN RECEIVED TWR PRIOR TO LNDG. HOWEVER WITH RVR 4000 FT WE SHOULD HAVE ASSUMED WET RWY CONDITIONS AND ASKED FOR BRAKING ACTION RPTS AND USED MEDIUM AUTO BRAKES. WE SHOULD ALSO HAVE REFUSED TWR'S REQUEST TO EXPEDITE TO THE END ASSUMING THE END OF THE RWY WOULD BE THE SLICKEST.