I have had decent luck with dawn dish soap and oxy clean .
for those of us who cant use dawn for allergic reaction
Ive had ENORMOUS succcess with a 20 dollar pipe-cleaner looking brush from a Farm and Fleet that I got for free, i can bend the end to a 90 degree and clean the top of the bottle(wire is stiff as hell, holds,) and soaking in some degreaser and pressure water.
I clean my cars reservoir like this twice, and it wasnt perfect but then I Put it back in use(Had a coolant refill coming up), and whatever was left came off in a month or so when I did it again.
the black and stuff visible was a minor bit on the outside, 90% or so is inside the reservoir, in the way right
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during 1st clean
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