Fluid doesnt evaporate, somewhere it exited the assembly if its low. It could be a check valve or valving seal issue inside the assembly allowing fluid to move through and air trap inside the hydraulic body. The fluid levels are a good starting point and it could just be age making the seals allow bypass of fluid and air pooling. Over filling the reservoir is a very bad decision, so be sure you dont do that as it will blow the pump seals.
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