I'm seeing lots of theories here,but one question.
Why did you remove the cowl vent to install the windshield in the first place??

I tend to agree with Lightholders Dad on this one.
I imagine hosing the water on would overwhelm the drain hence the leaks.
It might just be you are spraying too much water at an angle that the cowl vent wasn't designed to handle.
Just a thought.......