Depends what you are trying to remove - if it is just dirt build up then just use a mild car shampoo and a toothbrush and make sure you wash it off well. If you have lots of brake dust build up then you are best to use a standard brake cleaner solvent (something like Wurth Brake Cleaner, cheap and works well, although the solvent is the same in all of them).
Would advise against using any king of oil based cleaning agent, as these have a habit of migrating onto the discs and pads, not something that is recommended!!!! GT85 also contains PTFE, which would if it gets onto the pads would knacker them and seriously affect your braking ability.
A bit of elbow grease and soap is my best bet..........
Good idea, never thought of that, ill remember for next time, cheers amb67… by the way the muc off stuff i did buy is very good if anyone is looking for a specialist made for the job brake cleaner. Laterz everyone