The narrowest and at the same time longest path for the CSF flow is through the midbrain aqueduct from the third to fourth ventricle. Allegedly 1 mm diameter and 15 mm long. Entry and exit are funnel shaped. The flow is about 20 mL/hr. As the brain arteries expand during systole the CSF is pushed into the fourth ventricle. During diastolic relaxation some of the CSF is sucked back up. This turbulent to and fro - full of vortices with high flows - is great for flushing and purging - keeping our little bottle neck - pardon the pun - open and free of debris.

The aqueduct is not perfectly round and has some indentations. So the flow could look something like this.