What should be done before checking the free play of manual slack adjusters on s-cam brakes?

Before checking the free play of manual slack adjusters on s-cam brakes, it's essential to ensure that the vehicle is on level ground. This is important because a vehicle that is not level can create an inaccurate assessment of the slack adjuster's free play. Uneven surfaces can cause the brakes to engage or disengage improperly, leading to a false reading of whether the slack adjusters are functioning correctly. Level ground ensures that the weight of the vehicle is evenly distributed, which provides a more accurate check of the brake adjustment.

While the other steps might be relevant for overall safety and proper procedure, they don't directly affect the accuracy of measuring the slack adjusters. For instance, chocking the wheels and turning off the engine are safety measures that should be followed, and draining air pressure may be necessary for certain checks on air brakes, but neither of those steps addresses the specific requirement for ensuring the vehicle's position before measuring brake components.