On Sun, 15 Apr 2018 16:58:18 -0400, Retired > wrote:

>On 4/15/18 2:50 PM, wrote:
>> If you use the battery, chassis of the car, or master brake cylinder as an electrode, that is very problematic. You have voltages created by electrolysis with the master brake cylinder metal alloy. You have variations in the current path taken between the electrode and the reservoir. Finally, variations in the surface area of the conductor probe immersed in the reservoir.
>>
>> If you had two identical parallel electrodes of the same metal exposed to the brake fluid, then readings would be precise and repeatable,if not meaningful. The resistance reading would depend only on the electrode separation as well as the uninsulated length.
>>
>> Let's say you made such a test probe and calibrated it with fresh brake fluid that is mixed with various percentages of distilled water (0,1,2,3,4,&5%).
>>
>> Now you could put this calibrated probe into a random brake fluid reservoir and get a result everyone could agree on.
>>
>> It would probably provide an inaccurate reading due to soluble metals, salts and minerals contaminating the brake fluid. However that would only cause errors on the side of caution. If it gave a high enough resistance reading, that would indicate the reservoir was adequately dry.
>>
>> The next issue to worry about is: how fast does moisture migrate from wheel cylinders back to the master cylinder reservoir? I don't know. However, you could flush a small quantity from each bleeder valve and test that in some very small sample container directly in-line with the bleeder hose. Flush until the reading is good enough. This would be an excellent way to conserve on brake fluid, if that was a significant business expense.
>>
>> For readers that wonder why:
>> Water content is very bad for brakes. A hot brake cylinder will exceed the boiling point, turning water into steam.
>>
>> Brake fluid is designed to dissolve water and draw it away from the brake cylinders. The master brake cylinder is connected to the reservoir when the pedal is up, so this moisture is also diluted into that large volume as well.
>>
>
>Are you looking for something like this ?
>
>
>https://www.amazon.com/ABN-Automotiv...dp/B079V5N29M/

The problem with testing brake fluid is then you're exposing it to the atmosphere.
It'll soak up all the moisture available. Better to do what I do. Leave it in peace.