ACR Y ISSUED VISUAL APCH CLRNC BEFORE SIGHTING ACR X. LTSS. SYS ERROR. SITUATION: ATC EQUIP PROBLEM RADAR NON DETECTION OF ACFT BY TRACON RADAR RADAR TRACKING.

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ACR X WAS TURNED ONTO FINAL APCH COURSE RWY 27R AND CLRED FOR VISUAL APCH; ABOUT 8-10 MI FROM RWY. ACR Y WAS ON R DOWNWIND RWY 27R. ACR X RADAR TRACK DISAPPEARED FROM RADAR SCOPE AND PRIMARY TRACK BECAME DISPLACED S OF RWY 27R LOC. AFTER COMPLETING XMISSIONS TO 3 OR 4 OTHER UNINVOLVED ACFT (ABOUT 1 MIN); I TURNED ACR Y TO BASE; HAVING LOST TRACK OF AND POS ON ACR X. ACR Y RPTED ARPT IN SIGHT AND WAS CLRED FOR VISUAL APCH. IMMEDIATELY THEREAFTER I REALIZED THAT SOMETHING WAS AMISS. I OBSERVED A VERY FAINT PRIMARY TARGET JUST S OF LOC; 12 O'CLOCK AND 2 MI FROM ACR Y AT THIS POINT ACR Y ASKED IF HE WAS TO FOLLOW 12 O'CLOCK TFC TO WHICH I REPLIED IN THE AFFIRMATIVE HAVING REALIZED THAT I HAD INDEED LOST TRACK ON ACR X. ACR Y TURNED FINAL TO FOLLOW ACR X AND CONTINUED UNEVENTFULLY TO LNDG. I BELIEVE MY VIGILANCE WAS DISRUPTED DUE TO COMPLETE LOSS OF RADAR TRACKING ON AN ACFT UNDER MY CTL. THIS PROBLEM HAS BEEN A CONTINUOUS ONE; RPTED DAILY BY CTLRS AT O'HARE. IT OCCURS IN SEVERAL QUADRANTS ON THE RADAR DISPLAY A VARYING ALTS. OUR CONSIDERABLE WORKLOAD IS GREATLY INCREASED DUE TO EFFORT REQUIRED TO PROVIDE EXTRA VIGILANCE FOR FEAR OF 'LOSING' AN ACFT DURING SEQUENCING FOR APCH. WORKING WITH THIS FLAWED SYS CAUSES GREAT ANXIETY AND FRUSTRATION. I BELIEVE SUCH A PROBLEM REQUIRES IMMEDIATE AND EXTENSIVE ATTN.