Galvanic (dissimilar metals) voltage generation. The brake fluid is "grounded" through the metal lines etc. The fluid acts as an electrolyte while the voltmeter probes act as anode/cathode. Usually, if this is going on and you can measure it, there is some type of corrosiveness happening. If you're trying to figure out water content, you need to measure conductivity through resistance/impedence (ohms) by placing both probes in the fluid.
I do both conductivity and voltage. Conductivity (ohms) tells me how much moisture is present and vDC how much corrosion is present. I look for very high megaOhms and vDC less than 0.2. Those readings of voltages creeping up is due to polarization. YMMV. If you have any concerns- just change the fluid- its easy and inexpensive to do
When you test are you testing from battery +ve to brake fluid in the reservoir (voltage) and both probes in the fluid (Mohm)?
yes- for conductivity probes 1/2" apart in fluid. For vDC, one probe in fluid, the other on battery ground
