A B737-300 CREW; DURING LNDG ROLL AT ORD; EXPERIENCED POOR TO NIL BRAKING; RESULTING IN AN UNCOMFORTABLE; HIGHER THAN NORMAL SPD; TURN AT RWYS END.

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LNDG APCH ORD RWY 4R IN LIGHT RAIN/DRIZZLE. STABILIZED APCH AND NORMAL LNDG. CAPT LNDG. DURING BRAKING; TWR INSTRUCTED TO ROLL TO END AND EXPEDITE. I NORMALLY WOULD HAVE BRAKED MORE AGGRESSIVELY BUT I TRIED TO OBLIGE THE TWR'S INSTRUCTION. IN THE LAST 1000 FT OF RWY 4R; I APPLIED GREATER BRAKE PRESSURE BUT THE BRAKING ACTION WAS POOR TO INTERMITTENTLY NIL. I APPLIED MAX BRAKING AS WE APCHED THE END LIGHTS. THE FINAL TURNOFF IS A 90 DEG L TURN AND WE SUCCESSFULLY MADE THE TURN BUT AT A HIGHER RATE OF SPD THAN FELT COMFORTABLE. DURING THE FINAL 1000 FT OR SO; THE BRAKES RELEASED SEVERAL TIMES. I BELIEVE THE POOR BRAKING ACTION WAS POSSIBLY DUE TO RUBBER DEPOSITS ON RWY 4R; RWY MARKINGS AND RAIN. SUPPLEMENTAL INFO FROM ACN 562290: ON FINAL APCH TO RWY 4R ORD; TWR CTLR INSTRUCTED US TO ROLL TO END. CAPT WAS PF. APCH WAS STABILIZED; TOUCHDOWN; REVERSING; AND INITIAL ROLLOUT WERE NORMAL. DURING THE ROLLOUT; THE TWR SAID TO 'EXPEDITE TO THE END;' AS TFC WAS ON A ?? MI SHORT FINAL; WHICH WE TRIED TO DO. AT ABOUT 1500 FT FROM THE END; THE CAPT INCREASED BRAKING. WE REALIZED THAT BRAKING WAS POOR TO INTERMITTENTLY NIL. IT WAS MY IMPRESSION THAT THE GND SPD OF THE ACFT ACTUALLY INCREASED. THE BRAKES RELEASED SEVERAL TIMES IN THE LAST 1500 FT. THE TURNOFF AT THE END IS AN APPROX 90 DEG L TURN. THE TURN WAS SUCCESSFULLY EXECUTED AT A HIGHER SPD THAN NORMAL AND FELT QUITE UNCOMFORTABLE.