To check the free play of manual slack adjusters on S-cam brakes you should:

• A. Park on incline and apply the parking brakes
• B. Visually inspect the slack adjuster
• C. Start the engine and test brakes
• D. Park on level ground, chock the wheels, and release the parking brakes

Answer: (D)

Free play is the small amount of movement the slack adjuster push rod has before the brakes actually engage. For manual slack adjusters on S-cam brakes, you need the brakes to be released to measure this travel accurately.

Park on level ground, chock the wheels to prevent any movement, and release the parking brakes. This setup removes tension from the system (no air pressure holding the brakes or the parking brake applied), allowing you to feel or measure the free travel of the push rod. If you don’t release the brakes, you’ll get readings that aren’t true of the slack adjuster’s normal free play. Visual inspection alone won’t tell you how much travel occurs, and starting the engine or testing the brakes would inject air pressure and mislead the reading.