Make a grunge brush from scratch:

1.Open a new document,500x200,transperant background.Press "D" to set your colors back to ther default settings.

2.now Filter>Render>Cloudes
Filter>render>Difference Clouds
Image>Adjustments>Auto Levels
you should have something like this:


3.Create a new layer.

4.Now grab your gradient tool.
settings:Linear Gradient default setting.

5.Do a gradient from bottom to top.
should look something like this:


6.Now set the Blending Mode of the gradient layer to:"Color Dodge"
should look something like this:


7.Now duplicate the gradient layer.Layer>Duplicate Layer.

8.Now go to Edit>Transform>Rotate 180*
should look something like this:


9.Now got to Edit>Define Brush Preset.Now name your grunge brush and your done.



Here is what i made with my brushes: