Quest makes a couple of products designed for brush cleaning: Wally and Auto Dish. It is similar in cleaning power to Dawn but has a surfactant designed to break down quicker. Auto Dish has a fantastic orange smell. These are also good for the first step in wall cleaning where the drying aids/protectants are being applied and end up on the walls in tunnels and brush IBAs. IIRC They also have a couple of products that you can use online as a shampoo that uses a degreasing surfactant that will help keep the brushes cleaner.