What can cause the filling time of an air brake system to be longer than normal?

You answered correctly. The filling time of an air brake system can be longer than normal if there are leaking air tanks or connectors, or if the air compressor is not performing adequately. Leaks mean air escapes instead of building up pressure, and a weak compressor can't generate enough air flow efficiently. Too much water in the air tanks, however, does not directly increase the filling time, though it can cause other problems. In general, always check for air leaks and ensure the air compressor is working properly to keep the brake system functioning safely.