AN ACR B757 FLC HAS NUMEROUS FULL SCALE DEFLECTIONS TO THEIR LOC INDICATION DURING AN ILS APCH TO RWY 4R AT ORD.

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WE WERE VECTORED TO ILS RWY 4R ORD. AT 21 DME ON ORD VOR WE GOT FULL SCALE L DEFLECTION ON LOC. AFTER ABOUT 2 SECONDS IT CAME BACK TO CTR. WE CONTINUED THE APCH. AT 12 DME WE GOT FULL SCALE L DEFLECTION FOLLOWED BY FULL SCALE R DEFLECTION. FMC GAVE US AN AMBER WARNING LINE THROUGH LOC INDICATION AND WE GOT WARNING LIGHTS. WE TURNED AUTOPLT OFF AND FLT DIRECTOR OFF TO PREVENT ABRUPT FLT CTL INPUTS. WE ARE NOW FLYING RAW DATA AND WX IS 600 FT 2MI. WE CONTINUED THE APCH. AT 1100 FT AGL WE GET FULL SCALE DEV TO THE L. FO CONTINUES TO FLY NICE APCH USING MAP MODE FOR DIRECTION. LOC STAYS FULL L DEFLECTION TO 600 FT. WE BREAKOUT WELL R OF COURSE AND HAD TO CORRECT TO LINE UP WITH RWY. FROM 1100 FT TO 600 FT WE DID NOT KNOW IF WE SHOULD TURN L; GAR; OR IF LOC WOULD COME BACK TO CTR. I NOW BELIEVE WE HAD A R CORRECTION AND AFTER THE LAST SWING TO THE L WE DRIFTED R OF COURSE AND WHEN THE LOC RETURNED IT WAS AT FULL L DEFLECTION; BUT HOW WOULD WE KNOW THAT. USING AN ERRATIC LOC TO 600 FT; 2 MI IS UNACCEPTABLE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR IS A CAPT AND LINE INSTRUCTOR ON THE B757-200. ON THIS FLT HE WAS GIVING IOE TO A NEW FO. THE CAPT SAID THAT HE HAD SHOWN THE FO A COMPANY BULLETIN THAT WARNED OF THIS LOC PROB. DURING THE APCH HE POINTED OUT HOW DIFFICULT IT WOULD BE TO FLY A FAULTY LOC TO LOW MINIMUMS. NOW; HE SAID; HE REGRETS PUSHING THE APCH. HE SAID THAT AFTER THE FLT HE TALKED TO THE FO ABOUT THE APCH AND HOW HE; THE CAPT; SHOULD HAVE PERFORMED. HE ALSO SAID THAT HE TALKED TO THE CHIEF PLT AND DURING THIS CONVERSATION THE CHIEF PLT; APPARENTLY; CALLED A FACILITIES MGR AT ORD WHO ALLEGEDLY RESPONDED THAT NOBODY ELSE HAD EVER COMPLAINED ABOUT THE LOC. SO THE CHIEF PLT READ HIM THE COMPANY BULLETIN.