A couple of fundamental concepts here: 1) The Brush Tool must always have a brush assigned to it. If none is assigned, Adobe Illustrator will assign one. 2) Despite the fact that it is a setting in the Brushes panel, Basic is not a brush, it is the absence of a brush: a plain stroke.
Now, just to confuse the issue, the Pencil Tool does not require a brush, but it appears that the Control panel or the Pencil Tool preferences contains a bug. Unless "Keep selected" is activated in the Pencil Tool prefs, after creating a new path with Basic selected the Control panel Brush Definition field will show the last activated brush without applying it. Any additional paths created will be Basic, with whatever stroke weight you have selected.
So, if you want no brush applied, use the Pencil Tool set to Basic, and ignore the Control Panel if it shows a brush, or activate "Keep selected". Brush Tool ALWAYS defaults to a brush.
Peter
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