MD83 FLC CLBED AND DSNDED OFF ASSIGNED ALT IN RESPONSE TO TCASII RA ORIGINATING FROM AN ACFT WITH A MALFUNCTIONING MODE C.

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DEPARTING LAS ENRTE TO STL DIRECT CLRED BY PREVIOUS ZLA CTLR TO CLB TO OUR CRUISE ALT IN THE MID THIRTIES. WHEN CHANGED OVER TO NEXT LAX CTLR WAS TOLD TO LEVEL OFF AT 28000 FT (THIS CTLR'S TONE LED US TO BELIEVE SOMETHING WAS NOT NORMAL). WE PROMPTLY LEVELED AT 28000 FT. LAX CTLR THEN WAS POINTING TFC TO US; WHICH WE WERE UNABLE TO VISUALLY ACQUIRE. THE CTLR THEN ASKED THE CONFLICTING TFC TO CONFIRM HIS ALT; WHICH HE STATED 29000 FT. WE NEVER HAD A VISUAL CONFIRMATION ON THIS TFC. THE TCASII AT THIS TIME PICKED HIM UP AT 28000 FT. WE WERE NOT ABLE TO CONFIRM ALTS WITH LAX DUE TO RADIO TFC. OUR TCASII THEN GAVE US AN RA TO TAKE EVASIVE CLBS THEN A DSCNT TO AVOID TFC. SOON TO FOLLOW WAS AN ACR ACFT Z RESPONDING TO A TCASII RA FROM THIS SAME ACFT. WE NEVER DEVIATED MORE THAN 300 FT FROM ASSIGNED ALT. WHEN WE FINALLY CONTACTED ZLA AND TOLD THEM OF OUR RA; HE STATED THAT THE OTHER ACFT WAS ON A MAINT FLT. WE WERE NEVER TOLD OF THIS UNTIL AFTER WE HAD TO TAKE EVASIVE MANEUVERS SINCE WE COULD NEVER PICK UP THE ACFT VISUALLY. I COULDN'T BELIEVE THEY WOULD BE DOING A MAINT TEST FLT IN POSITIVE CTL AIRSPACE FOR AN INOP MODE C. THIS ACFT WAS GOING TO CAUSE HAVOC TO ALL TCASII EQUIPPED ACFT IN THAT VERY BUSY SECTION OF UNITED STATED AIRSPACE. I WAS LEFT NO OPTION BUT TO RESPOND TO MY TCASII SINCE I DID NOT KNOW THIS ACFT WAS ON A MAINT FLT AND NOT KNOWING HIS ALT. I FIGURE THE ACR Z CREW BEHIND US WAS ALSO FACED WITH THE SAME PREDICAMENT. THIS COULD HAVE BEEN A VERY SERIOUS EVENT DUE TO THE VIOLENT MANEUVERS THAT WERE REQUIRED FROM THE TCASII. WE WERE LUCKY TO STILL HAVE THE SEAT BELT SIGN ILLUMINATED AT THE TIME OF THIS EVENT. IF THIS SIGN WAS OFF WE WOULD HAVE HURT MANY PEOPLE.