working with only part of a brush

[celine5FBA]

I have a brush that looks like a pencil brush with parts that are light and you see the "brush hair"  and other parts heavier. It make a pattern and every time I use the brush you have all parts.

I want to "extract" the lighter part of this brush and create a new one. How can I do ?

[Monika Gause]

Please show the brush.