UNAUTH TKOF PROC WHEN BRAKING ACTION RPTED NIL.

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PRIOR TO DEP THE SO GOT ATIS AT TORONTO AND THERE WAS NO MENTION OF BRAKING ACTION FOR DEP RWY. AT APPROX XY05 LCL (NEW ATIS COMES OUT HRLY); THE FO GOT NEWEST ATIS AND THERE WAS NO MENTION ABOUT BRAKING ACTION BEING RPTED. ACFT WAS READY AND TOWED TO DEICING AREA. DURING MOVEMENT AND DURING DEICING THE FO WAS MONITORING GND CTL AND HEARD THE BRAKING ACTION RPT GIVEN FOR OUR DEP RWY. NEITHER THE CAPT OR SO HEARD THIS STATEMENT ABOUT BRAKING ACTION FOR A DEPARTING ACFT. SHE ASKED THE QUESTION TO THE CAPT ABOUT JBI (JAMES BRAKE INDEX) CHARTS AND DID NOT EXPRESS ANY VALUES. TKOF WAS MADE AFTER BEING DEICED AND GND/TWR NEVER MADE A GENERAL STATEMENT CONCERNING BRAKING ACTION ON THE DEP RWY. WE MADE THE TKOF JUST PRIOR TO THE NEW (HRLY) ATIS BEING GIVEN. AFTER TKOF; FO REQUESTED CAPT TO LISTEN TO NEWEST ATIS AND A BRAKING ACTION RPT WAS GIVEN THAT EQUATED TO 'NIL.' GND OR TWR SHOULD HAVE MADE ANNOUNCEMENTS AS SOON AS CONDITIONS CHANGED. NUMEROUS ACFT WERE DEPARTING ON OUR RWY. ALSO; BETTER COM WITHIN THE COCKPIT WHEN THE FO HEARD THE CHANGING BRAKING ACTION RPT. SUPPLEMENTAL INFO FROM ACN 264800: IT HAD BEEN LIGHTLY SNOWING ALL DAY. AIRPLANE WAS AT DEICING PAD WHEN TWR SENT A TRUCK DOWN THE RWY TO DETERMINE JBI. WHILE WE WERE DEICING; JBI WAS RPTED TO 2 OTHER ACFT AS .18. I ASKED CAPT WHAT JBI MEANT; AND IF WE HAD A CHART; ETC. HE ACTED UNCONCERNED; AND SO I DROPPED IT. TKOF WAS MADE ON RWY 15 WITH JBI OF .13. THIS EQUATES TO NIL IN OUR MANUALS. PROB AROSE BECAUSE TWR DID NOT SPECIFICALLY ANNOUNCE BRAKING ACTION. THEY ONLY SPOKE TO THE 2 ACFT AWAITING TKOF; WHICH I OVERHEARD. ATIS WAS NOT AMENDED UNTIL AFTER OUR TKOF. MY PERCEPTION WAS TO GO ALONG WITH UNCONCERNED ATTITUDE OF CAPT AND NOT PURSUE OR QUESTION.