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Check that the expanding rubber seal is in tacked. Evan a small hole will let air in and out with a change in barometric pressure. Over that period of time did the rubber seal follow the level down as the brake pads were wearing out?
I also agree that you should be changing the fluid more often, now you know there is a problem.
Not sure what method of testing you are using but it may be telling you percentage of water that the fluid can hold dissolved rather than percentage of the fluid by volume.
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