Final Answer:
When the brakes are hot - Using the parking brake when the brakes are hot can lead to damage and reduced effectiveness. Option C is the answer.
Step-by-step explanation:
Using the parking brake when the brakes are hot is advised against due to the risk of brake damage and reduced effectiveness. Hot brakes may lead to heat buildup, and engaging the parking brake can trap this heat, potentially causing brake components to warp or become less effective.
On steep hills, using the parking brake is recommended to prevent strain on the transmission. If the air pressure is low, it is crucial to avoid using the parking brake as it may not function properly.
However, the primary concern is applying the parking brake when the brakes are hot, making option C the correct choice for safe parking practices.