First, make a single example of the pattern you want: single dash; dash, dot; dot, dash, dot; whatever.
Marquee select around that, Edit > Define Brush Preset.
You will find your new brush appended to the end of the brushes list. Now, go to the Brushes palette (F5), and click on "Brush Tip Shape." Turn up the Spacing until you can see a dotted line at the frequency you want in the sample space. Then go to Shape Dynamics and under "Angle Jitter", change Control to Direction.