Final answer:
To adjust S-cam brakes with manual slack adjusters, the tractor parking brakes must be released. This allows for the accurate adjustment of the slack adjusters.
Step-by-step explanation:
When adjusting S-cam brakes equipped with manual slack adjusters, it is important to ensure that the tractor parking brakes must be released. This is because applying the parking brakes could mask any issues with the adjuster itself by keeping the slack adjuster from turning, preventing an accurate adjustment. Ensuring that the parking brakes are released allows for the manual slack adjusters to be turned properly, letting you adjust the brakes to the correct specification.