Not only would moisture enter due to leaks in the reservoir cover, but due to our rather damp climate it has been known to be absorbed from the air when you are filling the system. Try to do the job on a dry day, or at least don't leave the bottle open? Avoid reusing the fluid that you have flushed through when bleeding the system as that will have been more exposed. Also, don't use fluid form a part used bottle that has been hanging about for along time.
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