Why does contaminated brake fluid conduct electricity? Using a voltmeter to test for water contamination in brake fluid Contaminated brake fluid conducts electricity. In the automotive repair world, brake fluid is considered contaminated by two ways:

Brake fluid is hygroscopic. Higher water content in brake fluid means lower boiling point. Brake pads/rotors get hot which can boil the water near near the pads/rotors. Boiled water is steam which is a compressible gas. Having a gas in the brake lines leads to a spongy brake pedal. So, high water content absorbed by brake fluid is considered contaminated.
Brake lines contain copper. When microscopic bits copper get in the brake fluid, they can build up in the ABS unit causing failure of the ABS system. Thus, high amounts of copper mean contaminated brake fluid.

Since pure water and copper are both nonelectrolytes, how can brake fluid conduct electricity? Is there some other contaminant I am missing here that is an electrolyte? Is the water being absorbed, somehow, by the brake fluid not strictly H2O?
I am asking because I want to know if I can use a voltmeter to test if there is too much water contamination in brake fluid.
Even pure water is somewhat ionized. If water is more conductive than pure brake fluid, the test is valid. Why don't you try this as an experiment? To sample(s) of brake fluid fresh from a new, unopened, can, add distilled water, drop by drop, stirring each time and look for a change in conductivity. Measure the proportions, and you can graph the effect of moisture on conductivity.
N.B. There are two types of automotive hydraulic brake fluid in common use: glycol-based and silicone-based. You should test the conductivity of each, and the effect of moisture on each.
BTW, unless so much metal was suspended in the fluid to make a thick slurry, that would not affect conductivity. It is possible some metals, such as aluminum, would react with the fluid and ionize, but those metals should not be part of the brake system.