I don't use brushes for mud efects. I use photoreal samples of mud, dust, splatters, and so on...The brushes I use you can get them from free in the net, not that they are so rare. I first apply some general weathering with Multiply blending mode (opacity usually around 15-20%), then I apply some real (photoreal I mean) stains, dust, mud in Normal mode(opacity varies from 10 to 15%), once opacity reduced, brushes come to help, splatters, stains, etc. But if you want some nice effects go get some masked photoreal dirt pics, select portions, modify opacity, delete... They make the difference.I use filters to modify the Background focus and detailing,...Hue/Saturation command in PS it's of great help too.