The brake fluid tester pen measures the change of water content in brake oil based on the change in conductivity. It clearly shows the degree of water content through the LED. The high water content will cause the boiling point to decrease, affecting the braking performance, and also corroding the brake master cylinder and the sub-pump to cause rust and seizure. Red light indicates that the brake fluid needs to be replaced immediately, green light indicates good condition, and yellow light indicates that the condition is acceptable, but it is about to be replaced. Its operation is simple with accurate and fast testing. Green/yellow: The water content in the brake fluid is less than 1%, the brake fluid performance is good, and you can use it with confidence. Green/yellow/yellow/red/red: The