Final answer:
Brake slack on s-cam brakes should be checked by measuring the pushrod travel at each brake chamber with the vehicle parked on level ground, not by conducting a performance test like measuring stopping distance.
Step-by-step explanation:
To check the brake slack on s-cam brakes, it is not recommended to drive the vehicle at a specific speed and then apply the brakes to measure stopping distance. This method might be confused with a brake performance test, which is different from checking the brake slack. Instead, to check brake slack, the vehicle should be parked on level ground with the engine off and the wheels chocked. The correct procedure involves applying and releasing the brakes, and then measuring the pushrod travel at each brake chamber to ensure it is within specification. This measurement helps to determine if the slack adjusters are functioning properly and if the brake system is in good condition.