How do I select only a portion of a path to apply a brush stroke?

Hi

I'm trying to select a portion of a path by using the direct selection tool and shift clicking on the anchor points. But when I click and drag the brush over onto the path, the brush stroke is applied to the entire path..... which I dont want to happen. I only want it applied to the section of the path. It's driving me nuts! I must be doing something silly.