The brake fluid tester is a perfect digital measuring device for quickly and easily testing the presence of water in brake fluid, specially developed for automotive repair shops. Thanks to the 5 LEDs, the water content in the brake fluid from 0% to over 3% in 5 steps is accurately displayed. The brake fluid tester can be used for DOT 3/4/5 brake fluid. (Currently, cars' original brake fluids are DOT4+) Open the brake fluid reservoir and remove the tester cap. Turn on the tester by pressing the button on the top. The green LED starts to light up. Hold the test tubes in the brake fluid for one second until the plastic is in place. Then the test result appears on the LEDs. The tester automatically shuts off after 12 seconds. A battery included: a 1.5V, type AAANot included.