Why should the brake stroke be measured with the engine off?

Measuring the brake stroke with the engine off is crucial for ensuring accurate measurements. When the engine is running, the power brake system is active, which can lead to variations in brake performance, including pedal feel and stroke length. These variances may result from the application of vacuum assist in the braking system, influencing the measurement you are attempting to take. By conducting the measurement with the engine off, you eliminate these variables, allowing for a precise assessment of the brake stroke. Accurate measurement is vital for evaluating the condition and functionality of the braking system, which directly impacts vehicle safety.