Final answer:
To check the free play of manual slack adjusters on the S-cam brakes, you should park on a flat surface to ensure accurate measurements.
Step-by-step explanation:
To check the free play of manual slack adjusters on the S-cam brakes, you should park on a flat surface. This is because a flat surface will provide a stable and level ground for accurate measurements. Parking on a slope, hill, or curve may introduce variables that can affect the accuracy of the measurements.