You are probably talking about the Prelude from the first book of the Well-Tempered Clavier, but that's only a guess since you do not say.
One suggestion as to how to get the notes to have a more consistent dynamic is to work on just a single one of the arpeggios. Settle on your fingering. Then do completely the opposite of your goal. Play it a few times making the first note much louder than the others, then the second, then the third, etc. Then work on making the difference between the louder note and the others smaller and smaller, listening very carefully, until you are happy that they are equal in volume.
If that's not helpful at first, just work on one hand doing a scale CDEFGGFEDC, doing the same thing. Plan to emphasize one of the notes a lot, and then reduce the emphasis until it sounds equal to the others.
Give it a try. I often find that starting out doing the extreme opposite of what I want and then working towards what I want gradually helps my body figure out what it need to do to control the sound. This kind of control takes time to develop, so don't get frustrated.