NOT Brake Cleaner. Water, Spit,...heck...piss on it if things are that gunked-up...but NOT Brake Cleaner.
If you want to use a store-bought cleaner...ANY cheap non-Ammonia containing blue liquid spray window cleaner will cause BP fouling to run off freely with just a squirt or two. So will water...but NOT Brake Cleaner.
When I was a lot younger, and a lot more ignorant...I took my first muzzleloader afield with BP and went to shooting. I didn't know anyone who shot BP, and didn't know anything about "Fouling" buildup. I was a young man in my 20's and the solution to about anything that got "difficult"...like seating roundballs in a fouled barrel....was to just PUSH HARDER on the ramrod.
The bad news is that after a half dozen shots or so...each getting progressively more difficult to load...I left the range with a broken ramrod.
The good news is that I left that first range session without the sharp end of a broken ramrod imbedded in my arm.
Lessons learned.