You still are NOT telling us why you felt you "had to replace the brushes" ??? Just because they are showing sparks in NOT a reason to replace them.
Did they wear down to the springs, did they break..... WHAT made you feel they needed replacement?
If they did wear down within five hours, then perhaps there is something significantly wrong with the armature and the tool needs to go in to the service shop. But again, seeing "sparks" is NOT a reason to replace the brushes!