you may simply be reintroducing fouling back into the bore with the brush. after brushing just push a couple wet patches through to pull out the fouling until they come out mostly clean, then a dry patch or 2. if at this point you put the same brush back into the bore you just used to scrub it with, patches will come out looking dirty again, but you're not necessarily making the bore any cleaner. I do something like, wet patch, repeat wet patch until they don't look real dirty, then wet patch and let sit, then wet patch - it will look kind of dirty again, then solvent soaked brush, wet patch, again until mostly clean looking … then repeat solvent brushing or simply dry patch until they come out clean to remove all solvent and oil to finish.
