LEORX Brake Fluid Tester Vehicle Testing Tool with 5 LED Indicator for Auto Automotive Car 1. Turn on the brake fluid tester by pressing the button. A green light should light up. If the light does not come on, it may be time to change a battery. 2. Unscrew the lid for the brake fluid reservoir, and take off the cap for the brake fluid tester. 3. Dip the test probes into the brake fluid and the LED lights will indicate the percentage of water in the brake fluid. 4. After testing, remember to screw the lid of the brake fluid reservoir back on tightly. 5. The tester will automatically power off after 12 seconds of inactivity.