What could be a reason for prolonged air system pressure build-up time?

A dirty air filter can significantly impede the airflow into the air system, resulting in prolonged pressure build-up times. The air filter's primary function is to trap contaminants and particulates, allowing only clean air to enter the system. If the filter is clogged or dirty, it restricts airflow, decreasing the efficiency of the compressor. Consequently, the air system struggles to build up the necessary pressure for optimal brake function, leading to delays and potential operational issues.

The other options, while they may indicate problems that require attention, do not directly relate to the air system's ability to generate pressure. Excessive brake fluid pertains more to hydraulic systems and does not affect air pressure directly. Weak brake springs could lead to performance issues in brake engagement but do not influence the air system build-up time. Similarly, overheated coolant addresses engine cooling and is unrelated to air pressure dynamics in the braking system.