TotalTime action for Flash Video help

I'm trying to use totalTime action on MediaDisplay component in Flash 8 to
set the duration of the external FLV video to some value: the movie is 360
sec long, i tried to set its duration to 300 sec by applying the action
_root.myDisplay.totalTime = 300;

The controler component adapts to the new totalTime value and when the movie
reached 300 sec the slider stopped at the end of slider bar. BUT, the movie
still went on plying beyond 300 sec, and the slider just stood still at the
end point of the slider bar.

How can i REALY adjust the total playable time of the flv?

Thanks!