When preparing to measure slack adjustment, what is the first action you should take?

When preparing to measure slack adjustment in a heavy trailer's braking system, the first action should be to depress the locking sleeve and turn the adjusting bolt. This step is crucial because it allows for the proper adjustment of the brake shoes relative to the brake drum. By releasing the locking mechanism and adjusting the bolt, the technician can ensure that the slack in the brake assembly is accurately measured. This adjustment is essential for achieving optimal brake performance and ensuring safety on the road.

Other options, while they may be relevant to overall vehicle maintenance, do not directly pertain to the initial steps of measuring slack adjustment. Inspecting the brake fluid level and performing a visual inspection of the tires are good practices in general vehicle checks, but they do not specifically relate to the process of adjusting brake slack. Calibrating the brake system is a more complex procedure that typically follows adjustments rather than preceding them.