In which conditions should the trailer brake stroke be measured?

Measuring the trailer brake stroke is best done with the engine off and the brakes fully applied because this ensures that the brake system is under a static condition, allowing for the most accurate assessment of the brake stroke. When the brakes are fully applied, it allows the inspector to check how far the brake pushrod travels, which indicates the amount of force being applied by the brakes.

With the engine off, you eliminate any potential influence from the vehicle's hydraulic or pneumatic systems, ensuring that any measurement reflects the actual mechanical performance of the braking system itself. This condition gives a clearer picture of whether the brakes are functioning within the safe and permissible limits established by regulations.

The other options involve either having the engine running or the brakes released, which could lead to incorrect readings or misinterpretations of the brake stroke measurement. Having the brakes released would not provide any resistance, failing to measure the effective stopping capability of the brakes. Conversely, measuring with the engine running does not provide a stable environment for the brake components and could affect the reading due to pressure variations in the brake system.