I bought this tester on Ebay from China:http://www.amazon.com/Brake-Fluid-Tester...ke+fluid+tester
As you can see from the reviews, people are extremely satisfied with the tester.
I made a test in a bucket of water and it rapidly showed +4% water as it should. But then I tested some old brake fluid which has been on the shelf for 4 years. 4 years ago the container was opened and the plastic screw lid (with no seal) was put on again. It has been resting like this in a shed experiencing summers and cold winters and relative moistures going above 95%.
But my tester shows less than 1% water in the fluid. Which means the fluid should be OK. But can I trust the tester?
PS: I also tried the tester with the brake fluid in my cars brake fluid reservoir and again it showed less than 1%. But I do not know when it was changed the last time because I bought the car 6 months ago (used car).